J clin E&nlol Vol.43, No. 11, pp. 1123-1129, 1990 Rintcd in Great Britain.All rigbtsrescrvcd
0895-4356/90 33.00 + 0.00 Copyright 0 1990 Pcrgamon Pms pk
THE VALIDITY OF SINGLE-ITEM, SELF-ASSESSMENT QUESTIONS AS MEASURES OF ADULT PHYSICAL ACTIVITY THOMAS W. WEZ%,‘* CARL H. SLATBR,’LAWRIZNCE W. GREEN, VIRGINIAC. KENNEDY,’ DONNA L. ALBRIGHT’and CHIJAN-CHIJANWm?
‘The University of Texas Health Sciences Center at Houston, School of Public Health, 1200 Herman Pressler, P.O. Box 20186, Houston, TX 77225 and 2Henry J. Kaiser Family Foundation, Menlo Park, Calif., U.S.A. (Received in revised jòrm 2 Muy 1990)
Ab&mct-Individual energy expenditure (kcal/kg/day) was calculated from a detailed set of questions from the Health Promotion and Disease Prevention Supplement of the 1985 National Health Interview Survey. Responses to three single-item, self-assessment questions were compared to the energy expenditure variable to test criterion validity. Spearman’s correlation coefficient revealed moderate correlations between energy expenditure and corresponding levels of self-assessed leisure-time physical activity for each single-item question (r = 0.14 to 0.41). For purposes of measuring prevalente of physical activity, the energy expenditure variable was used to categorixe individuals into activity levels. The single-item questions were found to have Spearman’s correlations with the categorical measures ranging from 0.11 to 0.37 for leisure-time activities. Generally higher correlations were found for males and younger age groups (18-34 years). The relationships were interpreted as being weak mlative to an expected correlation of 0.75 for criterion vahdation. However, the single-item questions show promise for obtaining proxy estimates of the degree of leisure-time physical activity in a population. Physical activity lation coefficient
Criterion validation Energy expenditure National Health Interview Survey
INTRODUCI’ION
With the impetus of the Surgeon General’s report of 1979 entitled Healthy People [l], the Public Health Service set goals to be achieved by 1990 in five broad age groups affecting the health of Americans. In 1980, Promothg Health/Preventing Diiase [2] established 226 objectives among 15 priority areas, including 11 objectives for physical fitness and exercise. Two objectives to be achieved by 1990 concerned prevalente of adult physical activity, which is defined as any bodily movement pro*Author for correspondence. CE 4311I-*
Spearman’s corre-
duced by skeletal muscles that results in energy expenditure [3]. One objective concemed the prevalente of “vigorous physical activity” in adults (18-65 years) and a second objective referred to the prevalente of “appropriate physical activity” among older adults (65 years and over). Since the establishment of these two objectives, one problem has been the availability of national measures of the prevalente of adult physical activity that are accurate and feasible [4-111. There is as yet no agreement on a valid measure of physical activity, especially for adults and from survey data. LaPorte et al. [6], in an earlier review, identified 30 different tools
1123
1124
THOMAS W.
for measuring physical activity and provided an analysis of them in terms of several criteria. With regard to the survey instruments, their conclusion was that “little is known about their reliability and validity”, which could have implications for attempts to estimate the prevalente of physical activity in the population. The work of Stephens et al. [ 121confirmed the potential difficulty of assessing physical activity. They found prevalente estimates of leisure-time physical activity levels, as derived from different surveys, ranging from a low of 15% to a high of 78%. Slater et al. [lO] found the same problem when attention was restricted to National Center for Health Statistics (NCHS) surveys. Various questions used in three different NCHS surveys (NHANES 1, NHIS 1977 and NSPHPC) produced prevalente estimates for the limited physical activity levels of women of childbearing ages ranging from a low of 3.9% to a high of 39.1%. The genera1 problem with survey reliability and validity cited by LaPorte et al. [6] and the widely varying prevalente estimates noted by Stephens et al. [12] and Slater et al. [lO] leave several questions unanswered. The major question is the issue of validity, which can be assessed by giving different measuring instruments, one of which is considered a criterion, to the same set of respondents. This was done in the 1985 National Health Interview Survey (NHIS) Health Promotion/ Disease Prevention Supplement (HP/DP), which not only provided the different questions necessary for the comparisons, but also offered a more detailed set of questions (the presumed criterion set) for the assessment of physical activity [13]. This set of questions was designed to provide a total kilocalorie index of energy expenditure. The NCHS used a comprehensive list of leisure-time physical activities to aid recall, for the purpose of not only estimating prevalente of leisure-time physical activity, but to estimate the dose of the physical activity. Traditional single-item questions about physical activity, which were similar to those used in the NHANES, NHIS 1977 and the NSPHPC, were also included for comparative purposes. This set of data thus offers a single source for exploration of the validity of different physical activity survey measures-in particular, traditional single-item questions as compared to a more comprehensive and potentially criterion measure of leisure-time physical activity levels.
WEISSet al.
Selecting the best single-item question, if any, to retain in future surveys can be guided by the relative validity of the various single-item questions using the detailed question set as the criterion. Our study question then becomes “how valid are these single-item questions as measures of adult, leisure-time physical activity when compared to a composite score calculated from the detailed question set of the 1985 HP/DP Supplement?’ To answer this question, statistical analyses were performed to determine the criterion validity of the single-item questions [14]. That is, what is the degree of association between the single-item responses and the respondent’s activity level, as determined from the presumed criterion question set? A criterion validation study such as this one consists of measuring the degree to which selected measures (the single-item questions) agree with the criterion measure (the detailed question set). The criterion measure is not necessarily the “gold standard”, but an established and presumably better measure than the ones selected for comparison [15]. For the purpose of assessing the criterion validity of the single-item questions, the authors assumed the detailed question set of the HP/DP Supplement to be a criterion but do not necessarily assume it to be a “gold standard”. It should be noted that a great deal of effort on the part of the NCHS was put into this question set, which was based in large part on the work of Stephens and Craig [16] for the Canada Fitness Survey. METHODS
The 1985 Nutionul (NHIS)
Heulth Interview
Survey
The NHIS is an annual survey of the U.S. population based on interviews and is used to gather information about people’s health and their use of medical services. It is developed and implemented by the NCHS of the Public Health Service. The basic NHIS questionnaire varies little from year to year and includes information on demographics as wel1 as health care needs, health care utilization and health status. The survey includes the civilian, noninstitutionalized population of the U.S.A. The data is collected through personal, household interviews over a 1-year peroid. The NHIS is considered a complex, multistage area probability design and is typical in structure of national surveys of this type, such as past NHANES. This design de-
Physical Activity Measurement Validity
parts from simple random sampling and requines a set of sample weights for statistical analysis. A total of 36,399 eligible households (1 person/household) were chosen for the survey in 1985 of which 33,630 individuals participated, representing approx 90% of the eligible respondents. Further discussion of the survey design can be found in an earlier paper by Thornberry et al. [13]. HP/DP
aktailed question set
The basic NHIS questionnaire is supplemented each year with special additions such as the HP/DP Supplement in 1985. The HP/DP Supplement was designed to monitor progress toward the various health objectives for the nation initially described in Promoting Health/Preventing Disease [2]. The physical activity portion of the supplement contained a set of questions pertaining to 23 specific types of leisure-time physical activity, the frequency of the activity in the past 2 weeks, the duration in minutes and the perceived intensity. Categories of perceived intensity, as determined from heart rate or breathing, were large, moderate, small or none. Each question was read to the respondent by the interviewer. The individual activities and choices were offered by the interviewer. The specific activities for this survey are noted below in Table 1. Selfassessment
questions
The 3 single-item, self-assessment questions and an elaborated response set from the HP/DP Table 1. Activity categories of the HP/DP exercise section 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. ll. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
Walking for exercise Jogging or running Hiking Gardening or yard work Aerobics or aerobic dancing Other dancing Calisthenics or genera1 exercise Golf Tennis Bowling Biking Swimming or water exercises Yoga Weight lifting or training Basketball Baseball or softbal1 Football Soccer Volleyball Handball, racquetball or squash Skating Skiing Gther
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Supplement are listed below. The responses were not numerically scaled. Question t%. How much hard physical work is required on your job? Would you say a great deal, a moderate amount, a little or none? This question is referred to as Job Related Activity. Question &. How much hard physical work is required in your main daily activity? Would you say a great deal, a moderate amount, a little or none? This question is referred to as Muin Daily Activity. Question 5~. Would
you say that you are physically more active, less active or about as active as other persons your age? This question is referred to as Compared to Peers-3 L..evelsof response. Question Sb. Is that a lot more or a little more/a lot less or a little less active? This question is an elaboration on Question 5a.
Questions 6a and 6c were mutually exclusive. That is, only persons not in the workforce were asked to respond to Question 6c conceming the amount of physical work required in the main daily activity. In addition; Question 5b was not considered, by itself, to be a single-item question. It is an elaboration upon the responses from Question 5a. For the purpose of testing criterion validity and to see if additional information could be gained by a larger response set, the 4 responses from Question 5b were combined with the response “about the same” from Question 5a. This question is referred to as Compared to Peers-SLevels of response and is more detailed than Question 5a with its 3 response levels. Its 5 response levels become: a lot more; a little more; about the same; a little less; a lot less. A continuous variable representing energy expenditure (kcal/kg/day) can be computed from the detailed question set. In practica1 terms, this continuous variable can be used to categorize respondents into activity levels. This was done in the two previously published articles described below. The first prevalente estimate was based on methods used by Schoenbom [17], from here on referred to as Model 1. This model was specifically adapted from the Canada Fitness Survey for use with the HP/DP Supplement. Based on the composite energy expenditure score, individuals were categorized by Schoenbom (not the present authors) in the following manner: sedentary: 0.1-1.4 kcal/kg/day moderately active: 1.5-2.9 kcal/kg/day very active: 3.0 or more kcal/kg/day.
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TI-IOMAS w.
The second prevalente estimate (Model 2) was based on an analysis by Caspersen et al. [4]. Energy expenditure expressed in kcal/kg/hr was calculated from the detailed question set and converted to milliliters of oxygen/kg/min to compare with the respondent’s predicted maximum cardiorespiratory capacity. The respondent’s predicted maximum cardiorespiratory capacity was derived from regression equations specific to gender and using age as an independent variable [ 181.Respondents were grouped in the following manner corresponding to the 1990 objectives [4]: Sedentary. NO reported activity. Irregulurly active. One or more activities reported. The combined activities are performed for less than 20 minutes per session and less than 3 times per week. Regularly active not meeting the 1990 objectives. One or more activities reported. The com-
bined activities are performed for 20 min or more, 3 times or more per week. However, the activities do not require 60% or more of the respondent’s maximum cardiorespiratory capacity or they do not involve large muscle groups, or both (these are the requirements for meeting the 1990 objectives). Regularly active and meeting the 1990 objectives. In order to meet the 1990 objectives, a
respondent had to perform one or more activities for 20 min or more, 3 times a week or more. In addition, the activities had to require 60% or more of the respondent’s maximum cardiorespiratory capacity and had to involve large muscle groups. This category specifically measures progress towards the 1990 objectives relating to prevalente of adult physical activity. The requirement of 60% or more of maximum cardiorespiratory capacity is in line with the recommendations of the American College of Sports Medicine [ 191.
wEIS.
et ai.
Once classified into activity categories, respondents were stratified into the two age groupings specified by the 1990 objectives: 18-64 years; and 65 years and over. Further stratification by age was performed according to the NCHS breakdown of age groups in the survey. ANALYSIS
The first step in assessing criterion validity [14] was to compute Spearman’s correlation coefficient on the single-item responses and the continuous energy expenditure scores derived from the detailed question set (before categorizing into activity levels). Spearman’s r is equivalent to Pearson’s r except that values are ranked and correlations are performed on the ranks. It is a more appropriate statistic for ordinal data and reported slightly less conservative coefficients than Pearson’s r. Respondents were compared for the two age groups corresponding to the two 1990 objectives, stratified by gender and further stratified into 7 age groupings. The second step in assessing criterion validity was to correlate the single-item questions with the Models 1 and 2, which used the detailed question set to categorize individuals into activity levels. Spearman’s r values were calculated to measure the strength and direction of association between the single-item responses and the physical activity categories. RESULTS
The results of the first stage of the analysis are presented in Table 2, which shows the Spearman’s coefficients from correlating the singleitem question with the continuous energy expenditure variable derived from the detailed question set. Job Related Activity shows very
Table 2. Spearman’s correlation coefficients for single-item questions VS continuous measure; by two age groups and by gender
energy expenditure
Age wup W
Gender
Al1
18-64
65+
Male
Female
Question 6a: Job Related Activity
-0.02 n = 20,879
-0.02 n = 20,359
-0.05’ n =601
-0.05 n = 11,732
-0.02 n = 9120
Question 6c: Main Daily Activity Question 5a: Compared to Peers-3 Levels Combined (Questions Sa and 5b): Compared to Peers-5 Levels
0.22 n = 11,653 0.28 n = 32,822
0.19 n = 7227
0.21 n = 4399
0.31 n = 27,713 0.31 n = 27,439
0.27 n = 5079 0.28 n = 5018
*Not significant at the 0.05 level.
n =
0.29 32,415
0.30 3720
n =
0.30 n = 15,538 0.30 n = 15,346
0.22 n = 7861 0.25 n = 17,190 0.25 n = 16,971
Physical Activity Measurement Validity Table 3. Spearman’s
correlation
coefficients
for single-item questions bv aae strata
1127
VS continuous
energy expenditure
measure;
Agegroup 18-24
25-34
35-44
45-54
55-64
65-74
75+
Question 6az Job Related Activity
0.04 n =3562
0.02 n =6179
-0.10 n =4950
-0.10 n =3324
-0.09 n =2315
-0.03’ n = 506
-0.17’ n=96
Question 6c: Main Daily Activity
0.19 n = 1633
0.14 n = 1605
0.18 n=1061
n =968
n = 1911
0.20
0.15 n = 2572
0.17 n = 1865
0.40 n = 5313
0.33 n =7825
0.27 n =6039
0.29 n=4320
0.30 n=4236
0.31 n=3153
0.22 n=1982
0.41 n = 5258
0.34 n = 7724
0.28 n = 5966
0.29 n =4261
0.30 n =4190
0.32 n = 3119
n = 1947
Question 5a: Compared to Peers-3
Levels
Combined (Questions 5a and 5b): Compared to Peers-5 Levels
0.21
0.23
?? Not significant at the 0.05 level.
smal1 correlations (-0.02 to -0.05). Main Daily Activity shows moderate correlations ranging from 0.19 to 0.30. The male group tends to have slightly higher correlations. Compared to Peers-3 Levels shows more consistent scores across groups ranging from 0.25 to 0.3 1, while the combination of Questions 5a and Sb, Compared to Peers-5 Levels shows nearly the same degree of correlation, suggesting that little is gained by a more elaborated question. Table 3, with its 7 age strata, shows similar patterns. Looking at the peer comparison questions, however, it appears that for the younger age groups (18-24 and 25-34 years) the correlations tend to be higher. Due in part to the large sample sizes, these correlations are significantly different from those of the other age groups (p < 0.001). Not presented in the table are correlations within gender by age group for each single-item question. These were consistent with previous results (Tables 2 and 3) showing slightly higher correlations for males and for younger age groups. The results of the second stage of the analysis are found in Tables 4 and 5, which show
Spearman’s correlation coefficients from a comparison of the single-item responses to the categorized levels in Models 1 and 2. In Table 4, the correlation coefficients for Job Related Activity are al1 negative and are very smal1 (as expected) ranging from -0.10 to - 0.01. For Main Daily Activity, the coefficients range from 0.15 to 0.28. The male group shows relatively larger correlations than the other groups. Note also that the two models provided very similar coefficients. Compared to Peers-3 Levels, shows larger coefficients than the previous two questions ranging from 0.23 to 0.29. The consistency among groups is noteworthy. The combination of Questions 5a and 5b, Compared to Peers-5 Levels, showed similar correlations. The highest correlations were in the 18-64 yr age group and in the male group. This question, with 5 response levels has slightly higher correlations than Question 5a with its 3 response levels, but much higher than the questions relating to job activity and main daily activity. However, the addition of Question 5b gains little, if any, improvement in correlations
Table 4. Spearman’s correlation coefficients for single-item questions VScategorical activity measures; by two age groups and by gender Gender
Age group (yr) Al1
18-64
65+
Male
Female
Question 6a: Job Related Activity
Model 1 Model 2
-0.01 -0.06 n = 20,901
-0.02 -0.06
n =20,390
-0.04’ -0.10 n =611
-0.05 -0.09 n = 11,697
-0.01’ -0.02 n = 9250
Question 6cz Main Daily Activity
Model 1 Model 2
0.19 0.19 n = 11,615
0.15 0.17 n = 7199
0.17 0.16 n=4464
0.28 0.23 n = 3727
0.19 0.20 n = 8012
Model 1 Model 2
0.27 0.26 n = 32,818 0.28 0.26 n = 32,419
0.29 0.26
0.24 0.24 n = 5148
0.29 0.27 n = 15,503
0.29
0.25 0.25 n = 5086
0.29 0.28 n = 15,309
0.24 0.23 n = 17,462 0.25 0.23 n = 17,246
Question 5a: Compared to Peers-3
Levels
Combined (Questions 5a and 5b): Compared to Peers-5 Levels ?? Not significant at the 0.05 level.
Model 1 Model 2
n = 27,794
0.27 n = 27,446
THOMAS W. WEISSet al.
1128
Table 5. Spearman’s correlation coefficients for single-item questions VScategorical activity measures; by age strata Age group (yr) 18-24
25-34
3544
45-54
55-64
65-74
75+
Question 6a: Job Related Activity
Model 1 0.02’ 0.01* -0.07 -0.06 0.06 0.04’ Model 2 -0.02* -0.03 -0.10 -0.12 0.10 0.07’ n = 3544 n = 6180 n = 4934 n = 3285 n = 2308 n = 500
-O.lO* -0.26 n = 97
Question 6c: Main Daily Activity
Model 1 Model 2
0.18 0.11 0.20 0.12 n = 1643 n = 1593
0.15 0.15 n = 1851
Question 5a: Compared to Peers-3
Model 1 Model 2
0.36 0.30 0.28 0.34 0.27 0.26 n = 5308 n = 7817 n =6029
Model 1 Model 2
0.37 0.34 n = 5246
Levels
Combined (Questions 5a and 5b): Compared to Peers-5 Levels
n
0.31 0.28 = 7713
n
n
0.12 0.16 0.18 0.15 0.19 0.17 = 1064 n = 1101 n = 1926
0.29 0.27 = 5950
0.26 0.24 n =4309
n
0.26 0.25 =4252
n
0.16 0.13 =2638
n
0.27 0.27 =4251
0.28 0.21 0.28 0.20 n = 3205 n = 1958
n
0.28 0.30 0.22 0.27 0.29 0.21 = 4206 n = 3173 n = 1929
*Not significant at the 0.05 level.
over Question 5a, Compared to Peer+-3 Levels. With more strata, Table 5 shows about the same results except that the two younger age groups have slightly higher correlations.
DISCUSSION
An analysis using Spearman’s correlation coefficient to assess the validity of the single-item questions reveals that, as expected, job related activity appears to belong to a different domain than major daily activity, peer comparison and leisure-time physical activity, which behave as if they belong to the same domain. The correlation coefficients are generally < 0.40 for single-item questions relating to main daily activity and peer comparisons. These are considered moderate. However, they are interpreted as being weak relative to an expected standard of a correlation of 0.75 for the establishment of criterion validity. One drawback of a criterion validation study such as this is that validity itself is inferred, not measured [15] and that the conditions of a validation study are never exactly repeated. In addition, they assume that the criterion possesses validity and that the sample is representative of the population. Limitations of using the detailed question set as a criterion include problems of accuracy of a 2-week recall, reliability of judgments concerning activity intensity and relevante of listed activities for al1 sociodemographic groups. A 7-day recall period was found to provide useful estimates of habitual physical activity for research in epidemiologic and health education studies [20]. It is not known how reliable estimates are, based on the 1Cday recall of the detailed question set. In addition, the judgment
concerning activity intensity can also affect the validity of the question set. A possibility is that older persons may report higher intensity levels while those who possess higher levels of fitness report lower activity intensity. Lastly, since the respondents to the detailed question set encompass many different sociodemographic groups, it may be that the question set does not Capture specific activities of these groups. However, the 23 questions listed in Table 1 are quite comprehensive. In addition, a person had the option to report “other” as an activity, which was coded as 3,4 or 5 kcal/kg/hr depending on the leve1 of intensity. This compares to the energy expenditure for “walking for exercise”. For practica1 purposes, the single-item questions, with the exception of job related activity, suggest that these questions may provide rough estimates of energy expenditure levels in a population, although the detailed analysis by seven age strata suggests greater validity in younger age groups. Further study of the detailed question set is recommended, such as an observational study, to establish whether in fact the set may be treated as yielding a criterion variable for physical activity. REFERENCES U.S. Department of Health, Education and Welfare. Healthy People: The Surgeon General’s Report on Health Promotion and Disease Prevention, DHEW Public Health Service Publ. No. 79-55071. Washington, D.C.: U.S. Dept of Health. Mucation and Welfare; 1960. U.S. Dept of Health and Human Services. Promoting HeaWPreventine Dieease: Obiectives for tbe Nation. Washington, D.C.: U.S. Dept of Health and Human Services; 1980. Caspersen C., Powell K, Christenson G. Physical activity, exercise and physical fitness: definitions and distinctions for health-related research. Public Henlth Rep 1985; 100: 126-131.
Physical Activity Measurement Validity 4.
5.
6.
7.
8.
9.
10.
11.
12.
Caspersen C, Christenson G, Pollard R. Status of the 1990 physical fitness and exercise objective+evidence from NHIS 1985. PabIk HeaBh Rep 1986; 101(l): 581-592. Green L, Wilson R, Bauer K. Data requirements to measure progress on the objectives for the nation in health promotion and disease prevention. Am J PabIk Healtb 1983; 73(l): 18-24. LaPorte R, Monotye H, Caspersen C. Assessment of physical activity in epidemiologic research: problems and prospects. Pablic HeaRb Rep 1985; lOO(2): 131-147. Powell K, Spain K, Christenson G, Mollenkamp M. The status of the 1990 objectives for physical fitness and exercise. Public Health Rep 1986; 101(l): 15-21. Powell K, Paffenbarger R. Workshop on Epidemiologie and Public Health Aspects of Physical Activity and Exercise: a summary. Public Healtb Rep 1985; lOO(2): 118-126. Sallis J, Haskell W, Wood P, Fortmann S, Rogers T, Blair S, Paffenbarger R. Physical activity assessment methodology in the Five-City Project. Am J Epidemiol 1985; 121(l): 91-106. Slater C, Green L, Vemon S, Keith V. Problems in estimating the prevalente of physical activity from national surveys. Prev Med 1987; 16(l): 107-118. Wilson P, PatTenbarger R, Morris J, Havlik R. Assessment methods for physical activity and physical fitness in population studies: report of a NHLBI workshop. Am Heart J 1986; 1177-1192. Stephens T, Jacobs D, White C. A descriptive epidemi-
13. 14.
15.
16.
17. 18. 19.
20.
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ology of leisure-time physical activity. PubIk Healtlt Rep 1985; lOO(2): 147-158. Thomberry 0, Wilson R, Golden P. The 1985 Health Promotion and Disease Prevention Survey. Publk Health Rep 1986; lOl(6): 566-570. Brook RH. Ware JE Jr. Davies-Averv A. Stewart AL. Donald CÁ, Rogers WH, Williams KN, Johnston SA: Overview of adult health status measures fielded in RAND’s Health Insurance Study. Med Cw 1979; 17(7) Suppl: Chap. 5. American Psychological Association. Standar& for Educatienal and Psycbemglcal Tests. Washington, D.C.: APA; 1974. Stephens T, Craig CL. Fitness and activity measurement in the 1981 Canada Fitness Survey. Draft prepared for the NCHS Worksbep 011keeaing Pbyaical Fitstem and Aetlvlty Pattenm h General Ponalatlon Surveys, Arlie Ho&, Warrenton, Va; 2-5 June 1985. Schoenborn C. Health habits of U.S. adults. 1985: the “Alameda 7” revisited. Publlc Health Rep 1986; lOl(6): 571-580. Jones N, Campbell C. Cllnkal Exerclslng Testlng, 2nd edn. Philadelphia, Pa: WB Saunders; 1982. American College of Sports Medicine. Position statement: the recommended quantity and quality of exercise for developing and maintaining fitness in healthy adults. Med Sci Sports 1978; lO(3): vii-x. Blair SN, Haskell WL, Ho P, ‘Paffenbarger RS Jr, Vraniran KM. Farauhar JW. Wood PD. Assessment of habitual physical’activity by a seven-day recall in a community survey and controlled experiments. Am J Epidemie1 1985; 122(5): 794-804.
0895-4356/90 33.00 + 0.00 Copyright 0 1990 Pcrgamon Pms pk
THE VALIDITY OF SINGLE-ITEM, SELF-ASSESSMENT QUESTIONS AS MEASURES OF ADULT PHYSICAL ACTIVITY THOMAS W. WEZ%,‘* CARL H. SLATBR,’LAWRIZNCE W. GREEN, VIRGINIAC. KENNEDY,’ DONNA L. ALBRIGHT’and CHIJAN-CHIJANWm?
‘The University of Texas Health Sciences Center at Houston, School of Public Health, 1200 Herman Pressler, P.O. Box 20186, Houston, TX 77225 and 2Henry J. Kaiser Family Foundation, Menlo Park, Calif., U.S.A. (Received in revised jòrm 2 Muy 1990)
Ab&mct-Individual energy expenditure (kcal/kg/day) was calculated from a detailed set of questions from the Health Promotion and Disease Prevention Supplement of the 1985 National Health Interview Survey. Responses to three single-item, self-assessment questions were compared to the energy expenditure variable to test criterion validity. Spearman’s correlation coefficient revealed moderate correlations between energy expenditure and corresponding levels of self-assessed leisure-time physical activity for each single-item question (r = 0.14 to 0.41). For purposes of measuring prevalente of physical activity, the energy expenditure variable was used to categorixe individuals into activity levels. The single-item questions were found to have Spearman’s correlations with the categorical measures ranging from 0.11 to 0.37 for leisure-time activities. Generally higher correlations were found for males and younger age groups (18-34 years). The relationships were interpreted as being weak mlative to an expected correlation of 0.75 for criterion vahdation. However, the single-item questions show promise for obtaining proxy estimates of the degree of leisure-time physical activity in a population. Physical activity lation coefficient
Criterion validation Energy expenditure National Health Interview Survey
INTRODUCI’ION
With the impetus of the Surgeon General’s report of 1979 entitled Healthy People [l], the Public Health Service set goals to be achieved by 1990 in five broad age groups affecting the health of Americans. In 1980, Promothg Health/Preventing Diiase [2] established 226 objectives among 15 priority areas, including 11 objectives for physical fitness and exercise. Two objectives to be achieved by 1990 concerned prevalente of adult physical activity, which is defined as any bodily movement pro*Author for correspondence. CE 4311I-*
Spearman’s corre-
duced by skeletal muscles that results in energy expenditure [3]. One objective concemed the prevalente of “vigorous physical activity” in adults (18-65 years) and a second objective referred to the prevalente of “appropriate physical activity” among older adults (65 years and over). Since the establishment of these two objectives, one problem has been the availability of national measures of the prevalente of adult physical activity that are accurate and feasible [4-111. There is as yet no agreement on a valid measure of physical activity, especially for adults and from survey data. LaPorte et al. [6], in an earlier review, identified 30 different tools
1123
1124
THOMAS W.
for measuring physical activity and provided an analysis of them in terms of several criteria. With regard to the survey instruments, their conclusion was that “little is known about their reliability and validity”, which could have implications for attempts to estimate the prevalente of physical activity in the population. The work of Stephens et al. [ 121confirmed the potential difficulty of assessing physical activity. They found prevalente estimates of leisure-time physical activity levels, as derived from different surveys, ranging from a low of 15% to a high of 78%. Slater et al. [lO] found the same problem when attention was restricted to National Center for Health Statistics (NCHS) surveys. Various questions used in three different NCHS surveys (NHANES 1, NHIS 1977 and NSPHPC) produced prevalente estimates for the limited physical activity levels of women of childbearing ages ranging from a low of 3.9% to a high of 39.1%. The genera1 problem with survey reliability and validity cited by LaPorte et al. [6] and the widely varying prevalente estimates noted by Stephens et al. [12] and Slater et al. [lO] leave several questions unanswered. The major question is the issue of validity, which can be assessed by giving different measuring instruments, one of which is considered a criterion, to the same set of respondents. This was done in the 1985 National Health Interview Survey (NHIS) Health Promotion/ Disease Prevention Supplement (HP/DP), which not only provided the different questions necessary for the comparisons, but also offered a more detailed set of questions (the presumed criterion set) for the assessment of physical activity [13]. This set of questions was designed to provide a total kilocalorie index of energy expenditure. The NCHS used a comprehensive list of leisure-time physical activities to aid recall, for the purpose of not only estimating prevalente of leisure-time physical activity, but to estimate the dose of the physical activity. Traditional single-item questions about physical activity, which were similar to those used in the NHANES, NHIS 1977 and the NSPHPC, were also included for comparative purposes. This set of data thus offers a single source for exploration of the validity of different physical activity survey measures-in particular, traditional single-item questions as compared to a more comprehensive and potentially criterion measure of leisure-time physical activity levels.
WEISSet al.
Selecting the best single-item question, if any, to retain in future surveys can be guided by the relative validity of the various single-item questions using the detailed question set as the criterion. Our study question then becomes “how valid are these single-item questions as measures of adult, leisure-time physical activity when compared to a composite score calculated from the detailed question set of the 1985 HP/DP Supplement?’ To answer this question, statistical analyses were performed to determine the criterion validity of the single-item questions [14]. That is, what is the degree of association between the single-item responses and the respondent’s activity level, as determined from the presumed criterion question set? A criterion validation study such as this one consists of measuring the degree to which selected measures (the single-item questions) agree with the criterion measure (the detailed question set). The criterion measure is not necessarily the “gold standard”, but an established and presumably better measure than the ones selected for comparison [15]. For the purpose of assessing the criterion validity of the single-item questions, the authors assumed the detailed question set of the HP/DP Supplement to be a criterion but do not necessarily assume it to be a “gold standard”. It should be noted that a great deal of effort on the part of the NCHS was put into this question set, which was based in large part on the work of Stephens and Craig [16] for the Canada Fitness Survey. METHODS
The 1985 Nutionul (NHIS)
Heulth Interview
Survey
The NHIS is an annual survey of the U.S. population based on interviews and is used to gather information about people’s health and their use of medical services. It is developed and implemented by the NCHS of the Public Health Service. The basic NHIS questionnaire varies little from year to year and includes information on demographics as wel1 as health care needs, health care utilization and health status. The survey includes the civilian, noninstitutionalized population of the U.S.A. The data is collected through personal, household interviews over a 1-year peroid. The NHIS is considered a complex, multistage area probability design and is typical in structure of national surveys of this type, such as past NHANES. This design de-
Physical Activity Measurement Validity
parts from simple random sampling and requines a set of sample weights for statistical analysis. A total of 36,399 eligible households (1 person/household) were chosen for the survey in 1985 of which 33,630 individuals participated, representing approx 90% of the eligible respondents. Further discussion of the survey design can be found in an earlier paper by Thornberry et al. [13]. HP/DP
aktailed question set
The basic NHIS questionnaire is supplemented each year with special additions such as the HP/DP Supplement in 1985. The HP/DP Supplement was designed to monitor progress toward the various health objectives for the nation initially described in Promoting Health/Preventing Disease [2]. The physical activity portion of the supplement contained a set of questions pertaining to 23 specific types of leisure-time physical activity, the frequency of the activity in the past 2 weeks, the duration in minutes and the perceived intensity. Categories of perceived intensity, as determined from heart rate or breathing, were large, moderate, small or none. Each question was read to the respondent by the interviewer. The individual activities and choices were offered by the interviewer. The specific activities for this survey are noted below in Table 1. Selfassessment
questions
The 3 single-item, self-assessment questions and an elaborated response set from the HP/DP Table 1. Activity categories of the HP/DP exercise section 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. ll. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
Walking for exercise Jogging or running Hiking Gardening or yard work Aerobics or aerobic dancing Other dancing Calisthenics or genera1 exercise Golf Tennis Bowling Biking Swimming or water exercises Yoga Weight lifting or training Basketball Baseball or softbal1 Football Soccer Volleyball Handball, racquetball or squash Skating Skiing Gther
1125
Supplement are listed below. The responses were not numerically scaled. Question t%. How much hard physical work is required on your job? Would you say a great deal, a moderate amount, a little or none? This question is referred to as Job Related Activity. Question &. How much hard physical work is required in your main daily activity? Would you say a great deal, a moderate amount, a little or none? This question is referred to as Muin Daily Activity. Question 5~. Would
you say that you are physically more active, less active or about as active as other persons your age? This question is referred to as Compared to Peers-3 L..evelsof response. Question Sb. Is that a lot more or a little more/a lot less or a little less active? This question is an elaboration on Question 5a.
Questions 6a and 6c were mutually exclusive. That is, only persons not in the workforce were asked to respond to Question 6c conceming the amount of physical work required in the main daily activity. In addition; Question 5b was not considered, by itself, to be a single-item question. It is an elaboration upon the responses from Question 5a. For the purpose of testing criterion validity and to see if additional information could be gained by a larger response set, the 4 responses from Question 5b were combined with the response “about the same” from Question 5a. This question is referred to as Compared to Peers-SLevels of response and is more detailed than Question 5a with its 3 response levels. Its 5 response levels become: a lot more; a little more; about the same; a little less; a lot less. A continuous variable representing energy expenditure (kcal/kg/day) can be computed from the detailed question set. In practica1 terms, this continuous variable can be used to categorize respondents into activity levels. This was done in the two previously published articles described below. The first prevalente estimate was based on methods used by Schoenbom [17], from here on referred to as Model 1. This model was specifically adapted from the Canada Fitness Survey for use with the HP/DP Supplement. Based on the composite energy expenditure score, individuals were categorized by Schoenbom (not the present authors) in the following manner: sedentary: 0.1-1.4 kcal/kg/day moderately active: 1.5-2.9 kcal/kg/day very active: 3.0 or more kcal/kg/day.
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TI-IOMAS w.
The second prevalente estimate (Model 2) was based on an analysis by Caspersen et al. [4]. Energy expenditure expressed in kcal/kg/hr was calculated from the detailed question set and converted to milliliters of oxygen/kg/min to compare with the respondent’s predicted maximum cardiorespiratory capacity. The respondent’s predicted maximum cardiorespiratory capacity was derived from regression equations specific to gender and using age as an independent variable [ 181.Respondents were grouped in the following manner corresponding to the 1990 objectives [4]: Sedentary. NO reported activity. Irregulurly active. One or more activities reported. The combined activities are performed for less than 20 minutes per session and less than 3 times per week. Regularly active not meeting the 1990 objectives. One or more activities reported. The com-
bined activities are performed for 20 min or more, 3 times or more per week. However, the activities do not require 60% or more of the respondent’s maximum cardiorespiratory capacity or they do not involve large muscle groups, or both (these are the requirements for meeting the 1990 objectives). Regularly active and meeting the 1990 objectives. In order to meet the 1990 objectives, a
respondent had to perform one or more activities for 20 min or more, 3 times a week or more. In addition, the activities had to require 60% or more of the respondent’s maximum cardiorespiratory capacity and had to involve large muscle groups. This category specifically measures progress towards the 1990 objectives relating to prevalente of adult physical activity. The requirement of 60% or more of maximum cardiorespiratory capacity is in line with the recommendations of the American College of Sports Medicine [ 191.
wEIS.
et ai.
Once classified into activity categories, respondents were stratified into the two age groupings specified by the 1990 objectives: 18-64 years; and 65 years and over. Further stratification by age was performed according to the NCHS breakdown of age groups in the survey. ANALYSIS
The first step in assessing criterion validity [14] was to compute Spearman’s correlation coefficient on the single-item responses and the continuous energy expenditure scores derived from the detailed question set (before categorizing into activity levels). Spearman’s r is equivalent to Pearson’s r except that values are ranked and correlations are performed on the ranks. It is a more appropriate statistic for ordinal data and reported slightly less conservative coefficients than Pearson’s r. Respondents were compared for the two age groups corresponding to the two 1990 objectives, stratified by gender and further stratified into 7 age groupings. The second step in assessing criterion validity was to correlate the single-item questions with the Models 1 and 2, which used the detailed question set to categorize individuals into activity levels. Spearman’s r values were calculated to measure the strength and direction of association between the single-item responses and the physical activity categories. RESULTS
The results of the first stage of the analysis are presented in Table 2, which shows the Spearman’s coefficients from correlating the singleitem question with the continuous energy expenditure variable derived from the detailed question set. Job Related Activity shows very
Table 2. Spearman’s correlation coefficients for single-item questions VS continuous measure; by two age groups and by gender
energy expenditure
Age wup W
Gender
Al1
18-64
65+
Male
Female
Question 6a: Job Related Activity
-0.02 n = 20,879
-0.02 n = 20,359
-0.05’ n =601
-0.05 n = 11,732
-0.02 n = 9120
Question 6c: Main Daily Activity Question 5a: Compared to Peers-3 Levels Combined (Questions Sa and 5b): Compared to Peers-5 Levels
0.22 n = 11,653 0.28 n = 32,822
0.19 n = 7227
0.21 n = 4399
0.31 n = 27,713 0.31 n = 27,439
0.27 n = 5079 0.28 n = 5018
*Not significant at the 0.05 level.
n =
0.29 32,415
0.30 3720
n =
0.30 n = 15,538 0.30 n = 15,346
0.22 n = 7861 0.25 n = 17,190 0.25 n = 16,971
Physical Activity Measurement Validity Table 3. Spearman’s
correlation
coefficients
for single-item questions bv aae strata
1127
VS continuous
energy expenditure
measure;
Agegroup 18-24
25-34
35-44
45-54
55-64
65-74
75+
Question 6az Job Related Activity
0.04 n =3562
0.02 n =6179
-0.10 n =4950
-0.10 n =3324
-0.09 n =2315
-0.03’ n = 506
-0.17’ n=96
Question 6c: Main Daily Activity
0.19 n = 1633
0.14 n = 1605
0.18 n=1061
n =968
n = 1911
0.20
0.15 n = 2572
0.17 n = 1865
0.40 n = 5313
0.33 n =7825
0.27 n =6039
0.29 n=4320
0.30 n=4236
0.31 n=3153
0.22 n=1982
0.41 n = 5258
0.34 n = 7724
0.28 n = 5966
0.29 n =4261
0.30 n =4190
0.32 n = 3119
n = 1947
Question 5a: Compared to Peers-3
Levels
Combined (Questions 5a and 5b): Compared to Peers-5 Levels
0.21
0.23
?? Not significant at the 0.05 level.
smal1 correlations (-0.02 to -0.05). Main Daily Activity shows moderate correlations ranging from 0.19 to 0.30. The male group tends to have slightly higher correlations. Compared to Peers-3 Levels shows more consistent scores across groups ranging from 0.25 to 0.3 1, while the combination of Questions 5a and Sb, Compared to Peers-5 Levels shows nearly the same degree of correlation, suggesting that little is gained by a more elaborated question. Table 3, with its 7 age strata, shows similar patterns. Looking at the peer comparison questions, however, it appears that for the younger age groups (18-24 and 25-34 years) the correlations tend to be higher. Due in part to the large sample sizes, these correlations are significantly different from those of the other age groups (p < 0.001). Not presented in the table are correlations within gender by age group for each single-item question. These were consistent with previous results (Tables 2 and 3) showing slightly higher correlations for males and for younger age groups. The results of the second stage of the analysis are found in Tables 4 and 5, which show
Spearman’s correlation coefficients from a comparison of the single-item responses to the categorized levels in Models 1 and 2. In Table 4, the correlation coefficients for Job Related Activity are al1 negative and are very smal1 (as expected) ranging from -0.10 to - 0.01. For Main Daily Activity, the coefficients range from 0.15 to 0.28. The male group shows relatively larger correlations than the other groups. Note also that the two models provided very similar coefficients. Compared to Peers-3 Levels, shows larger coefficients than the previous two questions ranging from 0.23 to 0.29. The consistency among groups is noteworthy. The combination of Questions 5a and 5b, Compared to Peers-5 Levels, showed similar correlations. The highest correlations were in the 18-64 yr age group and in the male group. This question, with 5 response levels has slightly higher correlations than Question 5a with its 3 response levels, but much higher than the questions relating to job activity and main daily activity. However, the addition of Question 5b gains little, if any, improvement in correlations
Table 4. Spearman’s correlation coefficients for single-item questions VScategorical activity measures; by two age groups and by gender Gender
Age group (yr) Al1
18-64
65+
Male
Female
Question 6a: Job Related Activity
Model 1 Model 2
-0.01 -0.06 n = 20,901
-0.02 -0.06
n =20,390
-0.04’ -0.10 n =611
-0.05 -0.09 n = 11,697
-0.01’ -0.02 n = 9250
Question 6cz Main Daily Activity
Model 1 Model 2
0.19 0.19 n = 11,615
0.15 0.17 n = 7199
0.17 0.16 n=4464
0.28 0.23 n = 3727
0.19 0.20 n = 8012
Model 1 Model 2
0.27 0.26 n = 32,818 0.28 0.26 n = 32,419
0.29 0.26
0.24 0.24 n = 5148
0.29 0.27 n = 15,503
0.29
0.25 0.25 n = 5086
0.29 0.28 n = 15,309
0.24 0.23 n = 17,462 0.25 0.23 n = 17,246
Question 5a: Compared to Peers-3
Levels
Combined (Questions 5a and 5b): Compared to Peers-5 Levels ?? Not significant at the 0.05 level.
Model 1 Model 2
n = 27,794
0.27 n = 27,446
THOMAS W. WEISSet al.
1128
Table 5. Spearman’s correlation coefficients for single-item questions VScategorical activity measures; by age strata Age group (yr) 18-24
25-34
3544
45-54
55-64
65-74
75+
Question 6a: Job Related Activity
Model 1 0.02’ 0.01* -0.07 -0.06 0.06 0.04’ Model 2 -0.02* -0.03 -0.10 -0.12 0.10 0.07’ n = 3544 n = 6180 n = 4934 n = 3285 n = 2308 n = 500
-O.lO* -0.26 n = 97
Question 6c: Main Daily Activity
Model 1 Model 2
0.18 0.11 0.20 0.12 n = 1643 n = 1593
0.15 0.15 n = 1851
Question 5a: Compared to Peers-3
Model 1 Model 2
0.36 0.30 0.28 0.34 0.27 0.26 n = 5308 n = 7817 n =6029
Model 1 Model 2
0.37 0.34 n = 5246
Levels
Combined (Questions 5a and 5b): Compared to Peers-5 Levels
n
0.31 0.28 = 7713
n
n
0.12 0.16 0.18 0.15 0.19 0.17 = 1064 n = 1101 n = 1926
0.29 0.27 = 5950
0.26 0.24 n =4309
n
0.26 0.25 =4252
n
0.16 0.13 =2638
n
0.27 0.27 =4251
0.28 0.21 0.28 0.20 n = 3205 n = 1958
n
0.28 0.30 0.22 0.27 0.29 0.21 = 4206 n = 3173 n = 1929
*Not significant at the 0.05 level.
over Question 5a, Compared to Peer+-3 Levels. With more strata, Table 5 shows about the same results except that the two younger age groups have slightly higher correlations.
DISCUSSION
An analysis using Spearman’s correlation coefficient to assess the validity of the single-item questions reveals that, as expected, job related activity appears to belong to a different domain than major daily activity, peer comparison and leisure-time physical activity, which behave as if they belong to the same domain. The correlation coefficients are generally < 0.40 for single-item questions relating to main daily activity and peer comparisons. These are considered moderate. However, they are interpreted as being weak relative to an expected standard of a correlation of 0.75 for the establishment of criterion validity. One drawback of a criterion validation study such as this is that validity itself is inferred, not measured [15] and that the conditions of a validation study are never exactly repeated. In addition, they assume that the criterion possesses validity and that the sample is representative of the population. Limitations of using the detailed question set as a criterion include problems of accuracy of a 2-week recall, reliability of judgments concerning activity intensity and relevante of listed activities for al1 sociodemographic groups. A 7-day recall period was found to provide useful estimates of habitual physical activity for research in epidemiologic and health education studies [20]. It is not known how reliable estimates are, based on the 1Cday recall of the detailed question set. In addition, the judgment
concerning activity intensity can also affect the validity of the question set. A possibility is that older persons may report higher intensity levels while those who possess higher levels of fitness report lower activity intensity. Lastly, since the respondents to the detailed question set encompass many different sociodemographic groups, it may be that the question set does not Capture specific activities of these groups. However, the 23 questions listed in Table 1 are quite comprehensive. In addition, a person had the option to report “other” as an activity, which was coded as 3,4 or 5 kcal/kg/hr depending on the leve1 of intensity. This compares to the energy expenditure for “walking for exercise”. For practica1 purposes, the single-item questions, with the exception of job related activity, suggest that these questions may provide rough estimates of energy expenditure levels in a population, although the detailed analysis by seven age strata suggests greater validity in younger age groups. Further study of the detailed question set is recommended, such as an observational study, to establish whether in fact the set may be treated as yielding a criterion variable for physical activity. REFERENCES U.S. Department of Health, Education and Welfare. Healthy People: The Surgeon General’s Report on Health Promotion and Disease Prevention, DHEW Public Health Service Publ. No. 79-55071. Washington, D.C.: U.S. Dept of Health. Mucation and Welfare; 1960. U.S. Dept of Health and Human Services. Promoting HeaWPreventine Dieease: Obiectives for tbe Nation. Washington, D.C.: U.S. Dept of Health and Human Services; 1980. Caspersen C., Powell K, Christenson G. Physical activity, exercise and physical fitness: definitions and distinctions for health-related research. Public Henlth Rep 1985; 100: 126-131.
Physical Activity Measurement Validity 4.
5.
6.
7.
8.
9.
10.
11.
12.
Caspersen C, Christenson G, Pollard R. Status of the 1990 physical fitness and exercise objective+evidence from NHIS 1985. PabIk HeaBh Rep 1986; 101(l): 581-592. Green L, Wilson R, Bauer K. Data requirements to measure progress on the objectives for the nation in health promotion and disease prevention. Am J PabIk Healtb 1983; 73(l): 18-24. LaPorte R, Monotye H, Caspersen C. Assessment of physical activity in epidemiologic research: problems and prospects. Pablic HeaRb Rep 1985; lOO(2): 131-147. Powell K, Spain K, Christenson G, Mollenkamp M. The status of the 1990 objectives for physical fitness and exercise. Public Health Rep 1986; 101(l): 15-21. Powell K, Paffenbarger R. Workshop on Epidemiologie and Public Health Aspects of Physical Activity and Exercise: a summary. Public Healtb Rep 1985; lOO(2): 118-126. Sallis J, Haskell W, Wood P, Fortmann S, Rogers T, Blair S, Paffenbarger R. Physical activity assessment methodology in the Five-City Project. Am J Epidemiol 1985; 121(l): 91-106. Slater C, Green L, Vemon S, Keith V. Problems in estimating the prevalente of physical activity from national surveys. Prev Med 1987; 16(l): 107-118. Wilson P, PatTenbarger R, Morris J, Havlik R. Assessment methods for physical activity and physical fitness in population studies: report of a NHLBI workshop. Am Heart J 1986; 1177-1192. Stephens T, Jacobs D, White C. A descriptive epidemi-
13. 14.
15.
16.
17. 18. 19.
20.
1129
ology of leisure-time physical activity. PubIk Healtlt Rep 1985; lOO(2): 147-158. Thomberry 0, Wilson R, Golden P. The 1985 Health Promotion and Disease Prevention Survey. Publk Health Rep 1986; lOl(6): 566-570. Brook RH. Ware JE Jr. Davies-Averv A. Stewart AL. Donald CÁ, Rogers WH, Williams KN, Johnston SA: Overview of adult health status measures fielded in RAND’s Health Insurance Study. Med Cw 1979; 17(7) Suppl: Chap. 5. American Psychological Association. Standar& for Educatienal and Psycbemglcal Tests. Washington, D.C.: APA; 1974. Stephens T, Craig CL. Fitness and activity measurement in the 1981 Canada Fitness Survey. Draft prepared for the NCHS Worksbep 011keeaing Pbyaical Fitstem and Aetlvlty Pattenm h General Ponalatlon Surveys, Arlie Ho&, Warrenton, Va; 2-5 June 1985. Schoenborn C. Health habits of U.S. adults. 1985: the “Alameda 7” revisited. Publlc Health Rep 1986; lOl(6): 571-580. Jones N, Campbell C. Cllnkal Exerclslng Testlng, 2nd edn. Philadelphia, Pa: WB Saunders; 1982. American College of Sports Medicine. Position statement: the recommended quantity and quality of exercise for developing and maintaining fitness in healthy adults. Med Sci Sports 1978; lO(3): vii-x. Blair SN, Haskell WL, Ho P, ‘Paffenbarger RS Jr, Vraniran KM. Farauhar JW. Wood PD. Assessment of habitual physical’activity by a seven-day recall in a community survey and controlled experiments. Am J Epidemie1 1985; 122(5): 794-804.
