Journal of Gerontology 1979, Vol. 34, No. 2, 242-245
A Long-Term Longitudinal Study of Intellectual Ability of Older Adults: The Matter of Selective Subject Attrition1 Ilene C. Siegler, PhD,2 and Jack Botwinick, PhD3
has been reported in several sources, as, for example, one by Busse (1970). The subjects were men and women volunteers, 60 to 94 place in 1955 and the last took place in 1976, years selected to reflect the age, sex, ethnic, more than 20 years later; there were 11 test and socioeconomic characteristics of the older sessions in all. population in Durham, NC. The original This long-term study included many mea- sample was composed of 270 subjects, 246 of surements covering several disciplines. The whom completed the WAIS (Wechsler, 1955) present report is of one of these measurements and are included in this study. The 246 com— intellectual ability as measured by the WAIS prised 171 white subjects (86 men, 85 women) (Wechsler, 1955). In particular, the present and 75 black subjects (33 men and 42 women). report points to the problem of selective sub- The ratio of black to white subjects at the start ject attrition (e.g., Riegel et al., 1967; Riegel & of the study remained approximately the same Riegel, 1972) and how this relates to describing until the final testing (31 to 38%). In the main, age changes. The present report charts WAIS the same applied to the ratio of women to men scores across the test sessions for only those (about 52%), except during the last three test subjects who completed the WAIS during all sessions, when the percentage of women the sessions prior to the one under examina- started to rise (test 9 to 11, 56 to 61%). tion. In other words, the present study was The average time between the first and directed to the question of ability levels and second longitudinal observations was nearly subject attrition in longitudinal investigation. three years; the inter-test intervals from the second through the ninth longitudinal observation ranged from less than one to three years. METHOD The 10th and 1 lth test sessions were carried The subjects' description and the goals, out after 9 and 11 month intervals respectively. methods, design and administration of the (See Table 1). extensive multidisciplinary longitudinal study the early 1950s the first of two longitudinal studies was planned at Duke UniverIsityNaging (see Busse, 1970). The first testings took
RESULTS AND DISCUSSION 'This study was supported in part by a PHS research grant AG00364 (HD00668) to Duke Univ. Ctr. for the Study of Aging and Human Development, and training grant AG00030 to the Aging and Development Program, Dept. of Psychology. Washington Univ., St. Louis, MO 63130. 2 Ctr. for the Study of Aging and Human Development and Dept. of Psychiatry. Box 3003, Duke Univ. Medical Ctr., Durham, NC 27710. 3 Dept. of Psychology, Washington Univ., St. Louis, MO 63130.
242
The subjects of this study were categorized in three age groups: 60 to 64 years, 65 to 74 and 75 to 94 years in order to keep relationships between scores and attrition rates as independent of age as possible and still keep the num-
Downloaded from http://geronj.oxfordjournals.org/ at New York University on September 7, 2015
Two hundred and forty-six subjects aged 60 to 94 were tested with the WAIS at test session 1, the start of a longitudinal study which lasted approximately 20 years. The subject attrition from the first session to the last (session No. 11) was progressive and selective, leaving mainly the intellectually superior ones in the study. Two facts appeared particularly important: One, little intellectual decline was seen in these superior subjects until very late in life. Two, analyses based on such superior subjects without reference to the larger body of subject populations can lead to spurious generalizations about aging in general. Age decline in intellectual ability seen in more representative populations may not be apparent when analyzing data of select samples.
SELECTIVE ATTRITION
Table 1. Dates of Longitudinal Testing, Age of Subjects at the Start of Testing and the Number of Subjects (N) Completing the WAIS in All Tests Sessions Up to and Including the Session (No.) Indicated. Dates (Month/Year)
60-64 N
65-74 N
75-94 N
Total N
1 2 3 4 5 6 7 8 9 10 11
3/55- 5/59 9/59- 5/61 1/64- 3/65 10/66- 6/67 4/68-11/68 2/70- 6/70 1/72- 4/72 2/73- 6/73 4/74- 8/74 3/75- 7/75 3/76- 8/76
49 39 28 27 21 19 16 15 14 12 10
130 94 67 48 40 32 26 18 13 13 8
67 36 19 10 7 5
246 169 114 85 68 56 44 34 27 25 18
2 1 0 0 0
115110105 100
90 85 80 75
AGE (years) o • 65-74 A 75-94
70
nl1
2 3 4 5 6 7 8 9 Longitudinal Test Session
10 11
Fig. I. Mean intelligence test score at the time of first testing as a function of the number of longitudinal test sessions. Only subjects who were tested on all test sessions up to and including the abscissa test number are represented here. For example, subjects represented at test session 11 comprise only those who had been tested on 10 previous occasions.
ber of subjects significantly large during the later test sessions. Table 1 indicates the number of subjects (N) in these groups who had been tested at each prior session. The severe decline in N reflects the expected attrition due to death and illness. In addition, the attrition was due to two procedural requirements; one, that the whole WAIS had to be completed, and two, very importantly, that for a subject to be represented at any one time of measurement he had to have been tested during each of the previous times of measurement. Fig. 1 is of mean WAIS Full Scale scores made at test I by only those subjects remaining in the study to the time represented by the abscissa values. Note the general pattern of increasingly higher test 1 scores as the number of subjects diminishes with greater numbers of test sessions. The same type of figure was drawn for both the Verbal and the Performance parts of the WAIS (not shown here) with essentially the same findings — differential subject dropout such that with each subsequent testing, the subjects who scored higher at test time 1 remained in the study. Since subjects represented in the later longitudinal test sessions (number 11 in Fig. 1, for example) are also represented in earlier sessions, appropriate statistical analyses comparing mean performance across sessions are not possible. Accordingly, two f-tests were carried out bearing on only a small part of the selective attrition. Of those aged 60 to 64, Table 1 shows 10 subjects dropped out of the study from test session 1 to test session 2. The mean WAIS Full Scale scores of these 10 dropouts were compared to that of the 10 subjects of this age who remained in the study throughout (N = 10, see session 11 of Table 1). The respective means ± one standard deviation were 84.8 ± 37.3 and 105.2 ± 40.5; with t (18) = 1.17,p > .05. The corresponding analysis of those aged 65 to 74 was M = 69.0 ± 28.8, N = 36; M = 114.0 ± 17.8, N = 8. The /-test (3.96, df = 41) was highly significant at less than the .0003 confidence level. This latter test indicated what is visually apparent in Fig. 1, viz. the effect of selective subject attrition. The standard deviations, particularly those associated with the younger group, reflected the very large individual differences. It is noteworthy that among those in the younger group who dropped out after the first test session, two people (20%) were not tested again because they moved.
Downloaded from http://geronj.oxfordjournals.org/ at New York University on September 7, 2015
Test Session (No.)
243
SIEGLER AND BOTWINICK
244
10 O O
CO
100
90 0
60
66
72 78 68 74 80 Mean Age at Testing Time (Years)
86
Fig. 2. Subjects aged 60 to 64 (left half of figure) and 65 to 74 years (right half of figure) at the start of the study were tested over the course of approximately 20 years. The first 7, 10 and 15 years of longitudinal research are represented by solid circles; 20 years of longitudinal research is represented by dashed lines. Table 1 shows the Ns for each of the curves.
Only one person (2.8%) in the older group moved and so was not tested. Twenty percent in the younger group and 41.7% of the older group died after the first test session. These differences in moving and death percentages may account for the significant f-test associated with the older sample and not the younger one. Effort was made to determine the nature of longitudinal age change in test score with each succeeding session of testing. That is, test scores were plotted of those subjects who completed the WAIS at the first two test sessions, those who completed the first three test sessions and so on through the whole series of 11 test sessions. The mass of data plots made for visual confusion; to simplify presentation, only four longitudinal series of the 11 were selected. Fig. 2 presents the Full Scale WAIS scores for those subjects who completed three test sessions (an average of about seven years of longitudinal research), four test sessions (about 10 years), seven sessions (about 15
years) and 11 test sessions (about 20 years). The respective Ns may be seen in Table 1. The data of the oldest group, 75 to 94 years, are not represented here because of severe attrition over the course of years of longitudinal testing. In the main, the figure reflects two main points: One, except in very old age, the fall in score is not very great. Two, as longitudinal research progresses, and the initially less able drop out from the study, the decline in scores over time decreases. This is seen more clearly in the age group 60 to 64 years. Note the relatively flat functions for at least the first 12 years of the 15-year and 20-year curves as compared to the declines seen during the briefer seven and 10 year periods of research. (That is, compare the first six points of the upper two curves with all the points of the two lower curves.) In the older group, 65 to 74 years, the massive drop from test session 1 to test session 2 in the 20-year curve beclouds the stability in score seen from the second session (early 70s) to the late 80s.
Downloaded from http://geronj.oxfordjournals.org/ at New York University on September 7, 2015
CO
SELECTIVE ATTRITION
SUMMARY
The long-term multidisciplinary study of aging at Duke University included testing with the WAIS. The present report is of WAIS scores only, emphasizing the importance of selective subject attrition in understanding age changes in intellectual ability of the elderly. Two hundred and forty-six subjects, selected to reflect demographic characteristics of older people in Durham, NC, were tested at the start of the longitudinal study lasting approximately 20 years. There were 11 test sessions with mean inter-test intervals ranging from nine months to about three years. For the purpose of this report, the subjects were categorized into groups aged 60 to 64 years, 65 to 74
and 75 to 94. The subject attrition over the years was particularly severe with the oldest group, as might be expected, making it necessary to confine most of the analysis to the two younger groups. The data were looked at in two ways. First, the mean scores made at test session 1 were plotted for all subjects. Then these test session 1 scores were plotted for only those subjects who completed two test sessions; for only those who completed three test sessions, and so on to only those who completed the 11 test sessions. In the main, these test session 1 scores were higher with each succeeding longitudinal test. In other words, in the course of years, as subjects dropped out of the study, or as they were eliminated from analysis because they missed one or more longitudinal testings, the remaining subjects were seen to be of progressively higher intellectual ability. The second way the data were looked at was to focus on longitudinal age changes. Data of subjects completing seven years of longitudinal testing were compared with data based on 10, 15 and 20 years of longitudinal testing. As the testing was continued from seven to 20 years — as mainly the intellectually superior subjects remained and the others dropped out — age decline in intellectual ability was seen as minimal, except perhaps, in very late life. Another way of saying this is that intellectually superior older adults seem to maintain their intellectual abilities over many years. The less able ones, the broader, general populations, show more age change. REFERENCES
Busse, E. W. A physiological, psychological and sociological study of aging. In E. Palmore (Ed.), Normal Aging. Duke Univ. Press, Durham, NC, 1970. Riegel, K. F., & Riegel, R. M. Development, drop and death. Developmental Psychology, 1972, 6, 306-319. Riegel, K. F., Riegel, R. M., & Meyer, G. A study of the dropout rates in longitudinal research in aging and the prediction of death. Journal of Personality and Social Psychology, 1967, 5, 342-348. Wechsler, D. A. Manual for the Wechsler adult intelligence scale. The Psychological Corp., New York, 1955.
Downloaded from http://geronj.oxfordjournals.org/ at New York University on September 7, 2015
While continued longitudinal testing can thus obfuscate findings of age decline seen earlier in longitudinal investigation, Fig. 2 does highlight a most important fact. There is a sizeable proportion of old people, identified by their initial test superiority in intellectual ability, and by their continued presence as subjects in longitudinal study, who decline very little as old age advances, or decline not at all, except perhaps in extreme old age. Not shown here are plots similar to that seen in Fig. 2, drawn for both the WAIS Verbal and Performance sections. The Verbal plots, as expected, reflect little or no age decline in any of the functions represented in Fig. 2. The Performance scores do reflect decline but continued longitudinal testing seemingly has lesser obfuscating character. That is, continued longitudinal testing over 15 and 20 years reflects decline mainly during the first 10 years of testing. Overall, the data of this study point to the importance of distinguishing between subjects selected to begin longitudinal investigation and subjects who persevere — subjects who remain in the study after a long time — subjects who show up to be tested at every test session. It would seem that the more stringent the testing demands are on the subject, the more it is mainly the intellectually, and perhaps physically, superior who live up to those demands.
245
A Long-Term Longitudinal Study of Intellectual Ability of Older Adults: The Matter of Selective Subject Attrition1 Ilene C. Siegler, PhD,2 and Jack Botwinick, PhD3
has been reported in several sources, as, for example, one by Busse (1970). The subjects were men and women volunteers, 60 to 94 place in 1955 and the last took place in 1976, years selected to reflect the age, sex, ethnic, more than 20 years later; there were 11 test and socioeconomic characteristics of the older sessions in all. population in Durham, NC. The original This long-term study included many mea- sample was composed of 270 subjects, 246 of surements covering several disciplines. The whom completed the WAIS (Wechsler, 1955) present report is of one of these measurements and are included in this study. The 246 com— intellectual ability as measured by the WAIS prised 171 white subjects (86 men, 85 women) (Wechsler, 1955). In particular, the present and 75 black subjects (33 men and 42 women). report points to the problem of selective sub- The ratio of black to white subjects at the start ject attrition (e.g., Riegel et al., 1967; Riegel & of the study remained approximately the same Riegel, 1972) and how this relates to describing until the final testing (31 to 38%). In the main, age changes. The present report charts WAIS the same applied to the ratio of women to men scores across the test sessions for only those (about 52%), except during the last three test subjects who completed the WAIS during all sessions, when the percentage of women the sessions prior to the one under examina- started to rise (test 9 to 11, 56 to 61%). tion. In other words, the present study was The average time between the first and directed to the question of ability levels and second longitudinal observations was nearly subject attrition in longitudinal investigation. three years; the inter-test intervals from the second through the ninth longitudinal observation ranged from less than one to three years. METHOD The 10th and 1 lth test sessions were carried The subjects' description and the goals, out after 9 and 11 month intervals respectively. methods, design and administration of the (See Table 1). extensive multidisciplinary longitudinal study the early 1950s the first of two longitudinal studies was planned at Duke UniverIsityNaging (see Busse, 1970). The first testings took
RESULTS AND DISCUSSION 'This study was supported in part by a PHS research grant AG00364 (HD00668) to Duke Univ. Ctr. for the Study of Aging and Human Development, and training grant AG00030 to the Aging and Development Program, Dept. of Psychology. Washington Univ., St. Louis, MO 63130. 2 Ctr. for the Study of Aging and Human Development and Dept. of Psychiatry. Box 3003, Duke Univ. Medical Ctr., Durham, NC 27710. 3 Dept. of Psychology, Washington Univ., St. Louis, MO 63130.
242
The subjects of this study were categorized in three age groups: 60 to 64 years, 65 to 74 and 75 to 94 years in order to keep relationships between scores and attrition rates as independent of age as possible and still keep the num-
Downloaded from http://geronj.oxfordjournals.org/ at New York University on September 7, 2015
Two hundred and forty-six subjects aged 60 to 94 were tested with the WAIS at test session 1, the start of a longitudinal study which lasted approximately 20 years. The subject attrition from the first session to the last (session No. 11) was progressive and selective, leaving mainly the intellectually superior ones in the study. Two facts appeared particularly important: One, little intellectual decline was seen in these superior subjects until very late in life. Two, analyses based on such superior subjects without reference to the larger body of subject populations can lead to spurious generalizations about aging in general. Age decline in intellectual ability seen in more representative populations may not be apparent when analyzing data of select samples.
SELECTIVE ATTRITION
Table 1. Dates of Longitudinal Testing, Age of Subjects at the Start of Testing and the Number of Subjects (N) Completing the WAIS in All Tests Sessions Up to and Including the Session (No.) Indicated. Dates (Month/Year)
60-64 N
65-74 N
75-94 N
Total N
1 2 3 4 5 6 7 8 9 10 11
3/55- 5/59 9/59- 5/61 1/64- 3/65 10/66- 6/67 4/68-11/68 2/70- 6/70 1/72- 4/72 2/73- 6/73 4/74- 8/74 3/75- 7/75 3/76- 8/76
49 39 28 27 21 19 16 15 14 12 10
130 94 67 48 40 32 26 18 13 13 8
67 36 19 10 7 5
246 169 114 85 68 56 44 34 27 25 18
2 1 0 0 0
115110105 100
90 85 80 75
AGE (years) o • 65-74 A 75-94
70
nl1
2 3 4 5 6 7 8 9 Longitudinal Test Session
10 11
Fig. I. Mean intelligence test score at the time of first testing as a function of the number of longitudinal test sessions. Only subjects who were tested on all test sessions up to and including the abscissa test number are represented here. For example, subjects represented at test session 11 comprise only those who had been tested on 10 previous occasions.
ber of subjects significantly large during the later test sessions. Table 1 indicates the number of subjects (N) in these groups who had been tested at each prior session. The severe decline in N reflects the expected attrition due to death and illness. In addition, the attrition was due to two procedural requirements; one, that the whole WAIS had to be completed, and two, very importantly, that for a subject to be represented at any one time of measurement he had to have been tested during each of the previous times of measurement. Fig. 1 is of mean WAIS Full Scale scores made at test I by only those subjects remaining in the study to the time represented by the abscissa values. Note the general pattern of increasingly higher test 1 scores as the number of subjects diminishes with greater numbers of test sessions. The same type of figure was drawn for both the Verbal and the Performance parts of the WAIS (not shown here) with essentially the same findings — differential subject dropout such that with each subsequent testing, the subjects who scored higher at test time 1 remained in the study. Since subjects represented in the later longitudinal test sessions (number 11 in Fig. 1, for example) are also represented in earlier sessions, appropriate statistical analyses comparing mean performance across sessions are not possible. Accordingly, two f-tests were carried out bearing on only a small part of the selective attrition. Of those aged 60 to 64, Table 1 shows 10 subjects dropped out of the study from test session 1 to test session 2. The mean WAIS Full Scale scores of these 10 dropouts were compared to that of the 10 subjects of this age who remained in the study throughout (N = 10, see session 11 of Table 1). The respective means ± one standard deviation were 84.8 ± 37.3 and 105.2 ± 40.5; with t (18) = 1.17,p > .05. The corresponding analysis of those aged 65 to 74 was M = 69.0 ± 28.8, N = 36; M = 114.0 ± 17.8, N = 8. The /-test (3.96, df = 41) was highly significant at less than the .0003 confidence level. This latter test indicated what is visually apparent in Fig. 1, viz. the effect of selective subject attrition. The standard deviations, particularly those associated with the younger group, reflected the very large individual differences. It is noteworthy that among those in the younger group who dropped out after the first test session, two people (20%) were not tested again because they moved.
Downloaded from http://geronj.oxfordjournals.org/ at New York University on September 7, 2015
Test Session (No.)
243
SIEGLER AND BOTWINICK
244
10 O O
CO
100
90 0
60
66
72 78 68 74 80 Mean Age at Testing Time (Years)
86
Fig. 2. Subjects aged 60 to 64 (left half of figure) and 65 to 74 years (right half of figure) at the start of the study were tested over the course of approximately 20 years. The first 7, 10 and 15 years of longitudinal research are represented by solid circles; 20 years of longitudinal research is represented by dashed lines. Table 1 shows the Ns for each of the curves.
Only one person (2.8%) in the older group moved and so was not tested. Twenty percent in the younger group and 41.7% of the older group died after the first test session. These differences in moving and death percentages may account for the significant f-test associated with the older sample and not the younger one. Effort was made to determine the nature of longitudinal age change in test score with each succeeding session of testing. That is, test scores were plotted of those subjects who completed the WAIS at the first two test sessions, those who completed the first three test sessions and so on through the whole series of 11 test sessions. The mass of data plots made for visual confusion; to simplify presentation, only four longitudinal series of the 11 were selected. Fig. 2 presents the Full Scale WAIS scores for those subjects who completed three test sessions (an average of about seven years of longitudinal research), four test sessions (about 10 years), seven sessions (about 15
years) and 11 test sessions (about 20 years). The respective Ns may be seen in Table 1. The data of the oldest group, 75 to 94 years, are not represented here because of severe attrition over the course of years of longitudinal testing. In the main, the figure reflects two main points: One, except in very old age, the fall in score is not very great. Two, as longitudinal research progresses, and the initially less able drop out from the study, the decline in scores over time decreases. This is seen more clearly in the age group 60 to 64 years. Note the relatively flat functions for at least the first 12 years of the 15-year and 20-year curves as compared to the declines seen during the briefer seven and 10 year periods of research. (That is, compare the first six points of the upper two curves with all the points of the two lower curves.) In the older group, 65 to 74 years, the massive drop from test session 1 to test session 2 in the 20-year curve beclouds the stability in score seen from the second session (early 70s) to the late 80s.
Downloaded from http://geronj.oxfordjournals.org/ at New York University on September 7, 2015
CO
SELECTIVE ATTRITION
SUMMARY
The long-term multidisciplinary study of aging at Duke University included testing with the WAIS. The present report is of WAIS scores only, emphasizing the importance of selective subject attrition in understanding age changes in intellectual ability of the elderly. Two hundred and forty-six subjects, selected to reflect demographic characteristics of older people in Durham, NC, were tested at the start of the longitudinal study lasting approximately 20 years. There were 11 test sessions with mean inter-test intervals ranging from nine months to about three years. For the purpose of this report, the subjects were categorized into groups aged 60 to 64 years, 65 to 74
and 75 to 94. The subject attrition over the years was particularly severe with the oldest group, as might be expected, making it necessary to confine most of the analysis to the two younger groups. The data were looked at in two ways. First, the mean scores made at test session 1 were plotted for all subjects. Then these test session 1 scores were plotted for only those subjects who completed two test sessions; for only those who completed three test sessions, and so on to only those who completed the 11 test sessions. In the main, these test session 1 scores were higher with each succeeding longitudinal test. In other words, in the course of years, as subjects dropped out of the study, or as they were eliminated from analysis because they missed one or more longitudinal testings, the remaining subjects were seen to be of progressively higher intellectual ability. The second way the data were looked at was to focus on longitudinal age changes. Data of subjects completing seven years of longitudinal testing were compared with data based on 10, 15 and 20 years of longitudinal testing. As the testing was continued from seven to 20 years — as mainly the intellectually superior subjects remained and the others dropped out — age decline in intellectual ability was seen as minimal, except perhaps, in very late life. Another way of saying this is that intellectually superior older adults seem to maintain their intellectual abilities over many years. The less able ones, the broader, general populations, show more age change. REFERENCES
Busse, E. W. A physiological, psychological and sociological study of aging. In E. Palmore (Ed.), Normal Aging. Duke Univ. Press, Durham, NC, 1970. Riegel, K. F., & Riegel, R. M. Development, drop and death. Developmental Psychology, 1972, 6, 306-319. Riegel, K. F., Riegel, R. M., & Meyer, G. A study of the dropout rates in longitudinal research in aging and the prediction of death. Journal of Personality and Social Psychology, 1967, 5, 342-348. Wechsler, D. A. Manual for the Wechsler adult intelligence scale. The Psychological Corp., New York, 1955.
Downloaded from http://geronj.oxfordjournals.org/ at New York University on September 7, 2015
While continued longitudinal testing can thus obfuscate findings of age decline seen earlier in longitudinal investigation, Fig. 2 does highlight a most important fact. There is a sizeable proportion of old people, identified by their initial test superiority in intellectual ability, and by their continued presence as subjects in longitudinal study, who decline very little as old age advances, or decline not at all, except perhaps in extreme old age. Not shown here are plots similar to that seen in Fig. 2, drawn for both the WAIS Verbal and Performance sections. The Verbal plots, as expected, reflect little or no age decline in any of the functions represented in Fig. 2. The Performance scores do reflect decline but continued longitudinal testing seemingly has lesser obfuscating character. That is, continued longitudinal testing over 15 and 20 years reflects decline mainly during the first 10 years of testing. Overall, the data of this study point to the importance of distinguishing between subjects selected to begin longitudinal investigation and subjects who persevere — subjects who remain in the study after a long time — subjects who show up to be tested at every test session. It would seem that the more stringent the testing demands are on the subject, the more it is mainly the intellectually, and perhaps physically, superior who live up to those demands.
245
