Journal of Gerontology: SOCIAL SCIENCES

Copyright 1991 by The Gerontological Society of America

1991, Vol.46, No. 2.S71-83

Levels of Disability Among U.S. Adults With Arthritis Lois M. Verbrugge,1 James M. Lepkowski,2 and Lisa L. Konkol3

This article studies the excess levels of disability experienced by persons with arthritis, compared to persons without the disease. The data set is the Supplement on Aging (1984 National Health Interview Survey); it has information for a national probability sample ofcommunity-dwelling persons ages 55 + (N = 16,148). (1) Arthritis people have more difficulty in physical functions, personal care, and household care than do nonarthritis persons. The excess disability is greatest for physical functions (walking, reaching, stooping, etc.). Disabled arthritis people have especially high degrees of difficulty in physical activities that require endurance and strength. (2) Various models are tested for walking, grasping, shopping, and light housework to show how comorbidity propels disability for arthritis people and to show arthritis' own contribution to disability in the presence of other chronic conditions. Difficulties escalate for arthritis people when they have other concurrent conditions. These models affirm that arthritis has a pronounced effect on physical dysfunctions, but these are not readily translated into personal and household care problems. Apparently, arthritis people often make successful accommodations so their roles and daily activities are not seriously affected by the disease.

the 1960s and 1970s, life-threatening diseases were the INdominant focus of health research and health planning; the ultimate goal was to reduce disease progression and thereby delay death. In the 1980s, political and popular concerns began shifting toward quality of life rather than merely its quantity, and interest in disability in late life increased. Policymakers and scientific advisers now frequently discuss older adults' effective functioning and independent living; that is, how to keep people resident in the community with maximal ability to take care of their personal and household needs, to work, and to have close social ties. Although disability ensues from both fatal and nonfatal chronic conditions, the latter — diseases and sensory/structural impairments that bother but do not kill — are finally getting the research and policy attention they deserve (Myers, 1988; Verbrugge, 1989). Arthritis is the most prevalent chronic condition for middle-aged and older people, based on the National Health Interview Survey (self-reported diagnoses and conditions; Dawson and Adams, 1987; Verbrugge, 1987, 1989). By gender, arthritis is the leading condition for women of middle (45-64) and older (65-74, 75 + ) ages. It stands out very prominently for women, with prevalence rates 20-25 percent higher than their second-ranked condition, high blood pressure. For men, arthritis ranks first (ages 65-74) or second (45-64, 75 + ), and it has less preeminence among men's leading conditions. Given such high prevalence, it is not surprising that musculoskeletal symptoms such as pain, stiffness, and aching top the list of adults' daily symptoms or are close to the top, based on health diary surveys (Verbrugge, 1987, 1989; Verbrugge and Ascione, 1987). Musculoskeletal symptoms can be due to disease or to injury or overexertion; the proportion attributed to disease ("arthritis" is named most) tends to increase with age.

Arthritis is cited most often as the principal cause of role limitations by middle-aged and older women, and as the second-ranked cause of limitations (after diseases of heart) by men of those ages (LaPlante, 1988, Table 1; Verbrugge, 1989). The reason why arthritis is the leading limiting condition stems from the combination of two features: its very high prevalence together with its tendency to cause moderate disability among persons with the disease (Verbrugge, Lepkowski, and Imanaka, 1989). This article pinpoints dysfunctions most often associated with arthritis by comparing levels of physical, personal care, and household care disabilities for arthritis versus nonarthritis persons. We examined what happens to disability levels when arthritis occurs in conjunction with other chronic conditions, and we estimated arthritis' own contribution to disability in that situation. Our basic analytic strategy was to compare subgroups defined by arthritis and other chronic morbidity with statistical controls for several sociodemographic characteristics. The data set was the 1984 Supplement on Aging, which has information on community-dwelling persons aged 55 + in the U.S. Data Source and Variables The Supplement on Aging (SOA) accompanied the 1984 National Health Interview Survey (NHIS) conducted by the National Center for Health Statistics (NCHS). The SO A is based on a probability sample of the U.S. civilian noninstitutional population aged 55+ (Fitti and Kovar, 1987). Altogether, 16,148 persons were interviewed. The SO A has a complex probability sample. In this analysis, we adjust for disproportionate sampling and response by using weights provided by NCHS. Thus, our arthritis rates and disability levels are estimates for the national population. Complex variances that adjust for cluster sampling were computed for the arthritis prevalence rates S71

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'Institute of Gerontology, department of Biostatistics and Institute for Social Research, and 'Department of Health Behavior and Health Education, The University of Michigan.

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VERBRUGGE ET AL.

reported here, but not for other aspects of the analysis. Thus, standard errors for group differences and multivariate models are underestimated; we accommodated this by using p ^ .01 for testing statistical significance rather than the usual/? ^ .05.

Male

Female

F/M

Male: NW/W

Female: NW/W

Arthritis3 (SOA) White 55-64 65-74 75-84 85 +

28.8 39.6 39.5 35.4

44.2 52.8 57.2 55.1

1.54 1.33 .45 .56

1.26 1.14 1.02 1.52

1.04 1.16 1.21 0.98

Non-White 55-64 65-74 75-84 85 +

36.3 45.0 40.3 53.9

45.8 61.2 69.0 54.3

.26 .36 .71 .01

All Races 55-64 65-74 75-84 85 +

29.5 40.1 39.6 36.7

44.3 53.6 58.2 55.0

.50 .34 .47 .50

Total (55 + )

34.8

50.6

1.45 Survey, 1983—1985b

For comparison: National Health Interview Arthritis All Races 45-64 21.4 33.9 65-74 37.1 52.8 75-84 41.1 56.6 85 + 37.6 57.0

1.58 1.42 1.38 1.52

Osteoarthritis c (ICD 715,716.9) (SOA) All Races 27.4 40.7 55-64 38.0 49.8 65-74 75-84 37.7 55.7 35.4 53.1 85 +

1.49 1.31 1.65 1.50

55 +

32.7

47.1

Rheumatoid Arthritis (ICD 714.0) (SOA]1 All Races 1.0 2.1 55-64 0.8 2.3 65-74 1.0 1.3 75-84 0.6 1.6 85 + 55 +

0.9

2.0

1.44

2.19 2.96 1.32 2.73 2.22

Source: Supplement on Aging, 1984 National Health Interview Survey. Key: F/M = ratio of female to male rates; NW/W = ratio of non-White to White rates. ••Includes osteo, rheumatoid, and spinal forms, plus less common arthropathies. The ICD-9 codes (as adapted in National Center for Health Statistics, 1985) are 711 .b,0,9; 712.b,8,9; 714-716; 720.0; 721. (b denotes blank, when there are insufficient details to assign a fourth digit.) This span of codes is used routinely by NCHS for national rates of arthritis based on NHIS. Standard errors for the SOA rates, based on complex variances, are available on request. "•Unpublished data provided by NCHS. Published rates for other recent years are in Collins (1986) and Dawson and Adams (1987). Rates for rheumatoid arthritis and osteoarthritis based on 1983-85 NHIS are published in LaPlante (1988). 'Stated as "osteoarthritis" or "degenerative joint disease" (715) or as unspecified "arthritis" (716.9).

rise in rates in late life has resisted explanation; it might be due, in part, to institutionalization of persons with severe cases of osteoarthritis and to early mortality of people with rheumatoid arthritis (Abruzzo, 1982; Pincus et al., 1984).

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Arthritis. — In our analysis, arthritis status was determined from medically coded self-reports. This requires explanation: In the interview, chronic conditions are elicited in two ways — directly by questions about certain conditions, and indirectly by probes about the problems that have caused current or recent disability and medical care. For every condition named, a Condition Record is generated; further details about each condition are asked and entered on the Record. After the interview, medical coders review the Condition Records and assign the most specific International Classification of Diseases (ICD) code possible to each (National Center for Health Statistics, 1985). An ICD code for arthritis occurs in two basic situations: (a) when the respondent states that a physician or physician's assistant named the condition as "arthritis" (that label alone or a more specific one such as "osteoarthritis"), or (b) when a nondiagnosed respondent states "arthritis," and all additional details about the condition corroborate it. The ICD codes for arthritis comprise osteoarthritis, rheumatoid arthritis, spinal forms such as ankylosing spondylitis, and other rare forms. The survey's structure is complex, and it does not give uniform opportunities to respondents to report their conditions. This puts close constraints on researchers about whether specific conditions can be studied in individuallevel models (like those here). It is legitimate to study a specific condition for the total sample only if all respondents were asked direct questions about its presence. For the SO A, arthritis is legitimate because all persons were asked, "During the past 12 months, did you have arthritis of any kind or rheumatism?" (The term rheumatism is no longer used by rheumatologists, but it is still common lay terminology for joint symptoms.) Almost all people who said yes to this direct question have a Condition Record with ICD-coded arthritis (93.8%). The other 6.2 percent got ICD codes for soft tissue and other nonarthritic conditions. Similarly, almost all who said no do not have ICD-coded arthritis (99.6%). The other 0.4 percent named conditions elsewhere in the interview that the medical coders ultimately judged to be arthritis. Thus there is, in fact, close overlap of arthritis self-reports with formal medical coding; misclassification levels are just 5-6 percent. We have chosen to base our analysis on the medically coded arthritis, rather than the direct (yes/no) question, because it is the final best judgment of arthritis presence in this data set. Prevalence rates of arthritis based on the SOA are shown in Table 1. Over 40 percent (43.7%) of community dwellers aged 55 + in the U.S. have arthritis. Arthritis rates increase with age, up to the age group 85 + . Rates are higher for women than men by about 50 percent, and are higher for non-Whites than Whites. The decline in arthritis rates at very elderly ages is counter to radiographic and clinical evidence about the disease. It has been found in other studies, too (Felson et al., 1987; Forman, Malamet, and Kaplan, 1983; Lawrence et al., 1989, Table 4). The absence of a distinct

Table 1. Arthritis Prevalence Rates, U.S., 1984 (Rates Expressed as Percents)

DISABILITY AND ARTHRITIS

Physical and social disability. — Disability can be viewed in two basic ways: by presence or level of difficulty a person has doing an activity, or by dependence on another person to accomplish it. Scientists interested in the epidemiology of disability — namely, factors that cause disability in the general population or among persons with a target disease — prefer indicators of difficulty. Those interested in needs and costs for long-term care at home/institutions are more likely to focus on dependency. Our interests align with the first perspective. We want to know what kinds of disability typically ensue from arthritis; therefore all our dependent variables concern difficulty. Broadly defined, physical disability refers to problems the body has in producing force and appropriate motions for tasks. The tasks are considered situation-free; that is, they are fundamental physical actions used in many specific settings of daily life. Social disability concerns problems someone has to stay clean and alive, maintain a dwelling, do productive work, socialize with friends and relatives, participate in civic activities, and have pleasurable leisure. Social disability is situation-related; it refers to accomplishment of roles in an environment, whether private or public. (Further theoretical discussion of these points appears in Verbrugge, 1990.) In the SOA data, we can study physical disability at two levels of generality: difficulties in gross mobility (3 items) and in more localized motions and strength (10 items). For social disability, there are items about two main arenas of life: personal care activities (5 items) and household management activities (6 items). The first set are commonly called basic activities of daily living (ADL); the second, instrumental activities of daily living (IADL). We will refer to the items as ADL/IADL or "daily activities." The disability items are named and defined in Table 2. Missing data (not ascertained) on the disability items are typically just under one percent (range 0.4—2.8%). In our analysis, arthritis and nonarthritis groups are compared for presence of difficulty (yes/no) and for degree of difficulty among the disabled (if yes: some/a lot/unable). Comparative approach. — The analysis begins very sim-

ply, by comparing disability levels for persons with arthritis to persons without the disease. The two groups are then split into four according to overall morbidity, and statistical controls for sociodemographic characteristics are introduced. Later, we split groups still farther in order to compare arthritis and nonarthritis persons with equal levels of morbidity. Our mixture of sampling (subgroup) and statistical controls in the analysis is deliberate; we want to portray disability levels of population subgroups that clinicians may encounter in patient care, while also taking into account sociodemographic heterogeneity across them. Levels of Disability for Arthritis and Nonarthritis People For all indicators, disability is more prevalent among arthritis than nonarthritis people (Table 2; all p =£ .01). Ratios of arthritis to nonarthritis (A/A) tend to be larger for physical disabilities than ADL/IADL disabilities. This makes good theoretical sense: Physical limitations are very proximate to the disease; they are the earliest and most direct outcomes of arthritis. Social disabilities are more distant, and people often unconsciously make small adaptations to get the job done despite stiffness or pain. Thus, physical difficulties do not ramify promptly or necessarily ever to social ones. Considering the degree of difficulty among disabled people (Table 3; all items p ^ .01): (a) For ADLs, disabled arthritis people report "some" difficulty more often than their disabled peers without arthritis, who are more likely to report "unable." (b) The arthritis people are likely to state mild and moderate levels for I ADLs, too. (c) For physical activities, disabled arthritis people are somewhat more mobile than their nonarthritis peers, and their upper extremity problems are less pronounced. But they clearly have more substantial problems when endurance and strength are considered. See the lower extremity and heavy lift items; these activities involve the knee, a common site of arthropathy. Looking more closely at lifting: Whether they have arthritis or not, about one third of the people with heavy-lift trouble cannot lift even 10 pounds. Of those with light-lift trouble, arthritis people have milder difficulty than their nonarthritis peers. This confirms that it is heavy knee use that is most troublesome for the arthritis group, (d) On average, disabled arthritis people have more total physical problems but fewer social ones. Summing up, disabled arthritis people typically have milder trouble with ADL/IADL activities, walking, and upper extremity motions than their nonarthritis peers. But they have more severe difficulty in physical activities that require endurance and strength. This reflects the direct toll that arthritic diseases take on musculoskeletal capabilities. Arthritis, Comorbidity, and Disability If the arthritis group is more burdened with chronic illness than the nonarthritis group, this could help explain their very different levels of disability. The difference in overall health for the two groups is, in fact, quite striking: Arthritis people have on average 2.8 chronic conditions besides arthritis, for a total of 3.8. Nonarthritis people have on average 1.8 total chronic conditions. This sizable difference stems from heterogeneity in the nonarthritis group, which contains people

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The SOA rates for arthritis are similar to those produced by the regular National Health Interview Survey (middle of Table 1). Rates for osteoarthritis and rheumatoid arthritis from the SOA are also shown. Interview-based rates exceed rates based on x-rays and medical exams up to very elderly ages, when radiographs show almost universal evidence of arthritis. The excess for interviews may occur, in part, because symptomatic people excessively attribute symptoms to arthritis, and in part because genuine early arthritis processes are not detectable by clinical means. Furthermore, rates based on exams/x-rays usually refer to single sites such as hand or knee rather than "any site," and are thus inevitably lower than (whole) person-based rates. For a review of arthritis rates based on self-reports and medical evidence, see Lawrence et al. (1989). A review paper on how reported joint symptoms concur with medical diagnosis for given individuals is in preparation and can be requested from the first author.

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VERBRUGGE ET AL.

Table 2. Levels of Disability for Arthritis and Nonarthritis People* (U.S. Noninstitutionalized Persons Aged 55 + ) Total Mobility Any difficulty: Walking Getting outside

Motions and Strength"1 Any difficulty: Walking 1/4 mile Walking up 10 steps w/o rest Standing/being on feet 2 hours Sitting for 2 hours Stooping, crouching, kneeling Reaching up over head Reaching out as if to shake hands Using fingers to grasp Lifting/carrying up to 25 lb. Lifting/carrying up to 10 lb. Average no. motion/strength limitations (X) One or more limitations Personal Care (ADL)e Any difficulty: Bathing or showering Dressing Eating Getting in/out of bed/chair Using/getting to toilet Average no. ADL problems (X) One or more ADLs Household Management (IADL)f Any difficulty: Preparing own meals Shopping for personal items Managing money Using the telephone Doing heavy housework Doing light housework Average no. I ADL problems (X) One or more IADLs

A/A"

14.5% 6.8

22.4% 10.1

8.3% 4.2

2.70 2.40

1.0 1.9

1.3 2.4

0.8 1.6

1.63 1.49

23.6 20.2 28.2 10.7 32.9 13.9 2.1 9.5 30.4 11.4

34.8 30.4 41.9 17.2 49.8 22.0 3.3 16.5 43.8 17.0

14.9 12.1 17.4 5.5 19.6 7.6 1.2 4.0 20.0 7.0

2.34 2.51 2.41 3.10 2.54 2.89 2.68 4.15 2.19 2.41

1.81 48.6

2.74 68.3

1.09 33.4

2.51 2.04

7.3 4.9 1.3 6.7 3.0

10.6 7.2 1.6 10.8 4.1

4.8 3.1 1.1 3.5 2.0

2.22 2.30 1.49 3.11 2.01

0.23 11.2

0.34 17.3

0.14 6.5

2.39 2.66

4.9 7.9 3.2 3.1 19.1 5.1

6.8 11.2 3.8 3.7 29.2 7.1

3.5 5.4 2.8 2.7 11.2 3.6

1.94 2.09 1.33 1.38 2.61 1.98

0.43 21.3

0.62 31.8

0.29 13.1

2.12 2.43

"All arthritis vs nonarthritis differences are significant at p =s .01. b A/A is the ratio of arthritis to nonarthritis. c The mobility items are: "Because of a health or physical problem, do you have any difficulty — Walking?, Getting outside?'' (2 items); If Yes, degree of difficulty is queried:' 'By yourself and without using special equipment, how much difficulty do you have (ACTIVITY) — some, a lot, or are you unable to do it?; "Because of a health or physical problem, do you usually — Stay in bed all or most of the time?, Stay in a chair all or most of the time?'' (confinement, 1 item). d The motion/strength items are:' 'By yourself and not using aids, do you have any difficulty — Walking for a quarter of a mile (that is about 2 or 3 blocks)?, Walking up 10 steps without resting?, Standing or being on your feet for about 2 hours?, Sitting for about 2 hours?, Stooping, crouching, or kneeling?, Reaching up over your head?, Reaching out (as if to shake someone's hand)?, Using your fingers to grasp or handle?, Lifting or carrying something as heavy as 25 pounds (such as two full bags of groceries)? [If Yes to 25 pounds] Lifting or carrying something as heavy as 10 pounds?'' [If Yes to any item] "How much difficulty do you have (ACTIVITY) — some, a lot, or are you unable to do it?" •The ADL Items are: "Because of a physical or health problem, do you have any difficulty — Bathing or showering?, Dressing?, Eating?, Getting in and out of bed or chairs?, Using the toilet, including getting to the toilet?"; [If Yes] "By yourself and without using special equipment, how much difficulty do you have (ACTIVITY) — some, a lot, or are you unable to do it?' The IADL items are: "Because of a health or physical problem, do you have any difficulty — Preparing your own meals?, Shopping for personal items (such as toilet items or medicines)?, Managing your money (such as keeping track of expenses or paying bills)?, Using the telephone?, Doing heavy housework (like scrubbing floors, or washing windows)?, Doing light housework (like doing dishes, straightening up, or light cleaning)?"; [If Yes] "By yourself, how much difficulty do you have [as above]?"

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Confinement: Bed all/most of time Chair all/most of time

Arthr Nonarth (Percents except where noted)

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DISABILITY AND ARTHRITIS

Table 3. Levels of Difficulty Among Disabled Persons Percents

Ratio (A/A)

Some

A lot

Unable

Some

A lot

A A

55.0 50.2

35.4 32.6

9.6 17.2

1.10

1.09

.56

A A

41.8 32.7

30.3 26.6

27.9 40.7

1.28

1.14

.69

A A

34.9 37.7

24.3 20.0

40.8 42.3

0.93

1.22

0.96

Standing 2 hours

A A

34.6 40.2

27.3 20.3

38.1 39.5

0.86

1.34

0.96

Stooping, crouching, kneeling

A A

38.2 46.6

29.6 22.3

32.2 31.1

0.82

1.33

1.04

A A

56.1 49.7

28.7 27.3

15.2 23.0

1.13

1.05

0.66

A A

62.4 57.9

30.1 28.3

7.5 13.8

1.08

1.06

0.54

A A

23.7 29.0

18.9 16.3

57.4 54.3

0.82

1.16

1.05

A A

31.4 26.5

23.2 21.2

45.4 52.3

1.18

1.09

0.87

Mobility Walking Getting outside

Upper Extremity: Reaching out Using fingers to grasp Strength (involves lower & upper extremities): Lifting 25 lb. Lifting 10 lb. Average no. of motion/strength limitations (among people w/1 + ) (X) Personal Care (ADL) Dressing

A A

1.24

4.01 3.24

A A

66.6 52.3

17.6 21.0

15.8 26.7

1.27

0.84

0.59

Using toilet

A A

40.8 33.4

27.4 26.9

31.8 39.7

1.22

1.02

0.80

Average no. of ADLs (among people w/1 + ) (X)

A A

Household Management (IADL) Preparing meals

0.90

1.98 2.21

A A

34.9 21.0

22.9 16.7

42.2 62.3

1.66

1.37

0.68

Shopping

A A

21.3 14.7

21.1 16.8

57.6 68.5

1.45

1.26

0.84

Light housework

A A

28.3 17.1

22.1 13.9

49.6 69.0

1.65

1.59

0.72

Average number of IADLs (among people w/1 + ) (X)

A A

1.94 2.22

0.87

Notes. For each item, percent distribution of degree-of-difficulty for persons who said Yes to the initial question ("Do you have difficulty percents add to 100.0 in a row. A/A is the ratio of arthritis to nonarthritis. Data for selected items are shown; others available on request.

with zero chronic conditions (29%) as well as people who have chronic conditions other than arthritis (71%). There is important heterogeneity in the arthritis group as well; some people have only arthritis and no other health problem (14%), while others have arthritis plus other chronic conditions (86%). Recognizing this diversity, we continue the analysis with four distinct groups: zero chronic conditions, arthritis only, other chronic conditions only, and arthritis plus other

') is shown;

chronic conditions. Concretely, these are population groups of distinct interest to health scientists and rheumatologists. The percentage distribution of people in the four groups is: 16.5, 6.1, 39.8, and 37.6 percent. For brevity, we label the groups as: Zero CC, Arth Only, OthCC Only, and Arth Plus. Some heterogeneity remains across the groups. Their average number of chronic conditions is 0.0, 1.0, 2.5, and 4.3, respectively. They differ in sociodemographic characteristics: Age increases across them (in the order stated

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Motions and Strength Lower Extremity: Walking 1/4 mile

Unable

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VERBRUGGE ET AL.

Table 4. Differences in Disability for Four Groups8 % With Any Difficulty

Mobility Walking Getting outside

Personal Care (ADL) Dressing Using toilet No. of ADLs 1 or more 2 or more Household Management (IADL) Preparing meals Shopping Light housework No. of I ADLs 1 or more 2 or more

Arth Only (2)

OthCC Only (3)

Arth Plus (4)

(2-1) = C

(4-3) = D

0.2 0.1

4.9 2.1

11.6 5.9

25.1 11.2

4.7 2.0

13.5 5.3

+ 8.8 + 3.3

1.4 2.4 3.4 0.8 0.0 0.2 3.2 0.5

11.0 13.7 21.7 7.8 1.3 8.4 18.0 4.4

20.3 23.4 26.2 10.2 1.6 5.4 26.7 9.4

38.7 46.3 54.3 24.1 3.5 17.7 47.8 18.6

9.6 11.3 18.3 7.0 1.3 8.2 14.8 3.9

18.4 22.9 28.1 13.9 1.9 12.3 21.1 9.2

+ 8.8 + 11.6 + 9.8 + 6.9 + 0.6 + 4.1 + 6.3 + 5.3

8.7 2.8

39.2 21.9

43.7 30.8

73.1 59.4

30.5 19.1

29.4 28.6

-1.1 + 9.5

0.1 0.1

1.8 1.0

4.4 2.7

8.0 4.5

1.7 0.9

3.6 1.8

+ 1.9 + 0.9

0.3 0.0

4.5 2.0

8.9 4.9

19.4 10.0

4.2 2.0

10.5 5.1

+ 6.3 + 2.1

0.0 0.1 0.1

0.8 1.5 1.0

4.8 7.4 4.9

7.8 12.7 7.9

0.8 1.4 0.9

3.0 5.3 3.0

+ 2.2 + 3.9 + 2.1

0.6 0.1

7.7 2.0

18.3 8.3

35.7 14.6

7.1 1.9

17.4 8.3

+ 10.3 + 4.4

Difference of Differences D-C

a

Data for selected items are shown; others available on request.

above). The two arthritis groups are about two thirds female, and the nonarthritis groups are about half female. Race differences are small (non-Whites are slightly more common in the arthritis groups). We now ask: What happens when arthritis is added to the lives of people who have other chronic conditions (comorbidity), versus people who have no other health problems? Stated otherwise, what disability levels do arthritis people experience in different health contexts? Analytically, we are looking for a statistical interaction effect between arthritis and other chronic conditions, and we expect it to be strongly positive. Observed group differences. — We begin with crosstabulations of disability level (none, some, a lot, unable) by the four groups. Differences in percents with difficulty were computed for [Arth Only - Zero CC] and [Arth Plus - OthCC Only]. Call the first difference C and the second difference D; these indicate the "additional" effect of having arthritis. Subtracting D-C, we find that the difference-of-differences, or interaction effect, is always positive (Table 4; one exception; most p ^ .01). This means that the combination of arthritis plus other chronic conditions (being in the fourth group) gives a very special push to disability.

Four disability variables were selected for special scrutiny: walking, grasping, shopping, and light housework. They were chosen to reflect different facets of daily life: lower extremity function, upper extremity function, awayfrom-home activity, and at-home activity. The observed interaction effects for these four variables are shown in Figure 1. To investigate the interaction further, we redrew the graphs to portray the overall rise in disability across the four groups (Figure 2). Groups on the X-axis are arranged by ordinal rank with respect to total morbidity. For walking, shopping, and light housework, disability increases monotonically from the first to fourth group. There is an anomaly for grasping: People with just arthritis have more difficulty using their fingers to grasp/handle than do people with other (nonarthritis) chronic conditions. This is the only disability item in the SOA that has a nonmonotonic rise across the four groups. Grasping trouble is a very particular effect of arthritis, a disease commonly located in hands; it is seldom a consequence for other diseases. Testing models: chi-square analysis. — We performed Mantel-Haenszel analyses on the cross-tabulations for walk, grasp, shop, and light housework (Landis, Hey man, and

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Motions and Strength Walking 1/4 mile Standing 2 hours Stopping, crouching, kneeling Reaching up Reaching out Using fingers to grasp Lifting 25 lb. Lifting 10 lb. No. of motion/strength limitations 1 or more 2 or more

Difference

Zero CC (1)

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DISABILITY AND ARTHRITIS

WALK

GRASP

25

WALK

25 -. r

OTH CC 20. _

,GRASP

ia.

LGT HOUSEWK

NO OTH CC

NO ARTH

ZERO

ARTH ONLY

OTH CC ONLY

ARTH PLUS OTH CC

A.CC

A,CC

A,CC

A,CC

ARTH

SHOP

C

LGT HOUSEWK

Figure 2. Monotonic increases in disability across the four groups (with an exception for Grasp).

C

C A

A

A

A

Figure 1. Disability in different circumstances of arthritis and comorbidity.

Koch, 1978; Mantel and Haenszel, 1959). The analyses test various hypotheses about interactions and produce chisquare values indicating degree-of-fit for each hypothesis. An advantage of Mantel-Haenszel analysis is that a hypothesis is tested simultaneously within specific subpopulations, and the overall chi-square value produced represents an average across them. We created subtables for age-genderrace, and our results should be viewed as controlling for (adjusting across) them. We used the SAS programs CATMOD and FREQ. The numerical results are in Table 5; table notes explain the models tested and how to read the chi-square values. Here, we state the central conclusions: (1) The groups differ mostly in their average levels of disability (means), rather than by very different response patterns (shapes). (2) The mean differences are largely linear (additive) rather than nonlinear. Thus, one can score the groups' overall health levels as 0, 1, 2, 3 and capture much of the disability differences among them. (Stated another way, it means that viewing the X axis as interval is actually quite acceptable; we introduced it as ordinal.) (3) But linearity is not the complete story; there is pro-

nounced interaction as well. Arthritis operates differently in two comorbidity situations; disability in propelled in the presence of other conditions. [For readers familiar with Mantel-Haenszel analysis, statement (1) is based on comparing Columns I and II of Table 5; statement (2) comes from noting that Column III is similar to II; and statement (3) is from comparing Columns IV and V.] (4) The groups differ most in walking difficulty and least in light housework problems; this corroborates our visual conclusions for Figure 1. (5) Although we saw a nonmonotonic rise in grasping difficulty in Figure 2, this Y operates largely like the others. The anomaly portrayed occurs only for the category "any difficulty"; it does not ramify further in the response distribution. (6) The conclusions above are very pervasive in sociodemographic groups. The majority of age-gender-race subgroups (10 of 16; the largest ones) contribute strongly and consistently to the overall chi-squares; small groups such as White males 85 + have weaker patterns, but these contribute little overall. In sum, there are strong linear and nonlinear effects in the disability differences for the groups. So, there are two true sides to the story: Disability rises across the four population groups in line with their increasingly poor health situation. But there is a special thrust, or boost, to disability for people burdened by both arthritis and other chronic conditions. Testing models: logistic regressions. — To state that special boost in a more quantitative manner, we performed a series of logistic regressions with a cumulative logit model (responses: none, some, a lot, unable). Age, gender, and

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Table 5. Mantel-Haenszel Analyses of Disability Groups: Zero CC, Arth Only, OthCC Only, Arth Plus (df = 3) Responses: None, Some, A lot, Unable (df = 3) Covariates: Age x Gender x Race (16 subtables)" (Chi-square values are shown)b

(ID

Trend in Mean Scores (df = 1) (HI)

Interaction Variation (df = 3) (IV)

1122.6 931.8

836.0 676.5

788.6 653.8

186.5 172.1

101.0 91.0

Grasp No covariates W/ covariates

876.9 777.6

688.5 597.1

541.7 487.7

215.2 192.3

165.5 146.2

Shop No covariates W/ covariates

481.8 331.0

390.0 261.4

381.9 254.7

47.0 33.9

17.5 10.8

Light Housework No covariates W/ covariates

306.7 223.1

213.9 146.8

209.2 136.6

42.5 34.2

Difference in Mean Scores (df = 3)

Walk No covariates WV covariates

Interaction in Mean Scores (df = 1) (V)

2.3 (NS) 0.8 (NS)

a Age (55-64, 65-74, 75-84, 85 + ) , Gender (Male, Female), Race (White, Black; Other Race excluded in the Mantel-Haenszel analyses; 1.3% of sample). •"Explanation of chi-square columns: Column I tests for differences in shape of the response distribution for the four groups. The chi-square value is the total variation in the 4 groups x 4 responses table; it diminishes with covariates since some of the original group differences are due to sociodemographic heterogeneity. Column II tests for differences in means (Y) across the groups; it assumes interval scoring of Y (responses). Column III tests for a significant correlation of X and Y, i.e., how much differences in group means are due to linearity rather than nonlinearity. Interval scoring on both X (group) and Y (response) is assumed. Columns IV and V pertain to testing a specific interaction: the differential effect of Arthritis in two situations (no other CC, other CC). IV shows how much the interaction involves shapes of distributions, and V how much it involves means (slopes of curves in Figure 1). The tests involve pooling Zero CC with Arth Plus, and Arth Only with OthCC Only, then contrasting these two test groups. All chi-square values shown are significant atp =s .01 unless otherwise noted (NS).

race were control variables. The SAS program LOGIST was used. The group variable is the key predictor. Its scoring was varied to test three different models. First, we assumed a simple linear increase in "health trouble" across the four groups (continuous variable 0,1,2,3)- Arth Only is assumed to have twice the trouble of the Zero CC group; OthCC Only is assumed to have triple the trouble, etc. Second, we tested a main effects, or additive, model, allowing explicit effects for presence of arthritis (XI) and other chronic conditions (X2). It asserts that a person with both arthritis and other chronic conditions gets the disability effect of each, but nothing special for the fact they co-occur. Lastly, we tested a full model with interactions (3 dummy variables; Zero CC as reference group). It is inherently the best statistical representation of the data, and our interest is in seeing how close the other two models come to it. Results of the three models are shown in Figure 3. Odds ratios generated by the models are portrayed (beta coefficients available on request). An odds ratio indicates the additional risk of disability for a group compared to the reference group (whose odds are set at 1.00). The other numbers are interpreted as "x times more likely to be disabled" than the reference group, here people with zero chronic conditions. The interaction model is the upper curve; the main effects model is the lower curve; the linear model is shown by nonjoined dots (near the bottom). For walking, there is an over one hundred-fold increase in disability risk for people with both arthritis and other chronic conditions (odds ratio 113.3). This is much larger than the

46.5 odds ratio for the adjacent group, people with other (nonarthritis) chronic conditions. The main effects and linear models prove to be far from the truth; the first increases disability risk for the Arth Plus group by 26.6 (substantial, but much smaller than 113.3), and the second by 21.3. It is the difference between these odds — 113.3 versus 26.6 and 21.3 — that reveals the genuine and large disability burden incurred when arthritis people have other, concurrent conditions. The results for shopping, light housework, and grasping are very similar (Figure 3). Grasping maintains its unusual feature of lower odds for the OthCC group than for the Arth Only group. Three technical comments: (a) It is unusual in survey research to see odds ratios so big (Figure 3), yet they are entirely correct. From one perspective, the odds ratio for Arth Plus tells us the great disability toll this group suffers compared to the other three. From another perspective, it gives insight into the "super healthiness" of people aged 55+ who have no chronic conditions (Zero CC, the reference group for odds calculations). (b) The Mantel-Haenszel and regression results have different emphases. In the first, the interaction component appears modest relative to the additive one, but in the second it is very large indeed. This difference occurs because the Mantel-Haenszel chi-squares compare results on a logit scale, while the regression odds ratios restore original scales. Both analyses are correct, providing different views of the full story. (c) The cumulative logit model performs a series of regressions on various dichotomous outcomes; in our case,

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Total Variation (df = 9) (I)

DISABILITY AND ARTHRITIS

120,

S79

WALK . . , , » INTERACTION H3.3 / MOOEL

100

GRASP

•0.

60.

MAIN EFFECTS (ADDITIVE) MODEL 20 • LINEAR MODEL

LOO ZERO CC

ARTHONLY

OTHCC ONLY

ARTH PLUS OTH CC

SHOP

ZERO CC

ARTHONLY

OTHCC ONLY

ARTH PLUS OTH CC

LGTHOUSEWK 70.81

ZERO CC

ARTHONLY

OTHCC ONLY

ARTH PLUS OTH CC

ZERO CC

ARTH ONLY

OTH CC ONLY

ARTH PLUS OTH CC

Figure 3. Odds ratios for three models: linear, additive, and interaction.

combining and contrasting none/some/a lot/unable three different ways. It assumes parallelism, that the increase in odds from successive levels of lower to higher disability (the 3 splits) is proportionately the same across subgroups (age, gender, race). For further insight into the disability process, we explored that assumption for walking, using the SAS program CATMOD. Parallelism does not hold across groups: Zero CC shows a shallow slope of increasing odds across levels of difficulty (thus, close to parallel), OthCC Only has a steeper slope, and the two arthritis groups have steepest and essentially equal slopes (memo on results is available). Thus, arthritis has a distinctive propelling effect on disability. Scrutiny of the slopes and response proportions shows that arthritis tends especially to boost mild and

moderate levels of difficulty. This additional analysis reveals that, if anything, our odds ratios for Arth Only and Arth Plus are underestimated. Summing up, when arthritis joins other chronic conditions, walking is severely compromised. Disability in other physical and social activities is also notably exacerbated for arthritis people who have comorbidities. Our analysis format (chi-square and Mantel-Haenszel) could be used to study other specific chronic conditions, and some of those analyses would reveal positive interaction effects (see Verbrugge, Lepkowski, and Imanaka, 1989, for clues). We have chosen arthritis because of its prominence in mid- and late life, not because we expect the boosting (interaction) effect for that disease only. Our aim is to show

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Table 6. The Specific Impact of Arthritis3 The specific impact is found in each row, by comparing adjusted values Percent With Any Difficulty Adjusted Percentsb

Observed Percents Nonarthritis

Arthritis

Arthritis

Nonarthritis

(A-A)

Walk 4.9 9.9 16.9 35.9

Zero CC 1 OthCC 2 OthCC 3 OthCC 4 + OthCC

0.3 2.2 7.0 12.9 31.2

5.4 10.2 16.8 34.4

1.6 2.9 7.3 12.6 30.0

— + 2.5 + 2.9 + 4.2 + 4.4

Only +1 OthCC +2 OthCC + 3 + OthCC

8.3 9.4 11.4 24.5

Zero CC 1 OthCC 2 OthCC 3 OthCC 4 + OthCC

0.3 1.3 2.6 6.5 14.8

8.0 9.2 11.3 24.3

0.3 1.3 2.8 6.8 15.2

— + 6.7 + 6.4 + 4.5 + 9.1

Only +1 OthCC +2 OthCC + 3 + OthCC

1.5 3.4 6.2 19.8

Zero CC 1 OthCC 2 OthCC 3 OthCC 4+OthCC

0.1 1.3 4.0 8.2 20.9

2.1 3.7 6.1 18.0

1.8 2.3 4.5 8.0 19.5

— -0.2 -0.8 -1.9 -1.5

Only +1 OthCC +2 OthCC + 3 + OthCC

1.2 2.3 3.3 12.8

Zero CC 1 OthCC 2 OthCC 3 OthCC 4 + OthCC

0.1 0.9 2.9 5.3 13.9

1.5 2.4 3.2 11.7

1.0 1.4 3.2 5.1 13.1

— + 0.1 -0.8 -1.9 -1.4

Grasp

Shop

Light Housework

Average effect of Arthritis based on Y = f [Age, Gender, Race, Total CC, Arthritis]' Net deviation from grand mean

Walk Grasp Shop Light Housework

Adjusted mean (percent)

Arthritis

Nonarthritis

Arthritis

+ .013 + .034 -.012 -.010

-.010 -.027 + .009 + .007

.158 .129 .068 .042

Nonarthritis > > <

Levels of disability among U.S. adults with arthritis.

This article studies the excess levels of disability experienced by persons with arthritis, compared to persons without the disease. The data set is t...
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