In most countries, population estimates

Transcription

1 Proxy Reporting and the Increasing Prevalence of Arthritis in Canada Anthony V. Perruccio, MHSc 1,2 Elizabeth M. Badley, DPhil 1,2 ABSTRACT Background: Analyses of the 1994/95 to 1998/99 Canadian National Population Health Surveys (NPHS) reveal significant, and greater than projected, increases in the reported arthritis in the household population aged 15+ years, from 13.4 to 16.0%. The objectives of this study were to determine whether the increasing prevalence of arthritis can be explained by a) changes in the age and sex structure of the population, or b) the variation in the proportion of proxy respondents and whether proxy reporting affects the overall prevalence estimate. Methods: Overall analyses of 1994/95, 1996/97 and 1998/99 cycles of the NPHS, for arthritis or rheumatism reported as a long-term health problem diagnosed by a health professional. Stratified analyses, by respondent type, to account for the decreasing proportion of proxy respondents over time (33% to 16%). Results: Overall age-sex standardized prevalence estimates were similar to crude estimates. The crude prevalence of arthritis in proxy respondents was stable (approximately 8.5%), whereas in self-respondents it increased from 15.8 to 17.4% over the 3 cycles. Adjustment for the lower prevalence in proxy respondents increased the estimated overall prevalence of arthritis by at least 1 percentage point for each cycle year. The disparity between self- and proxy reporting was higher for younger people and females. Conclusion: Significant disparity exists in age- and sex-specific prevalence estimates between self- and proxy respondents. The increase in prevalence of arthritis over time is a result of increased reporting by self-respondents. Proxy reporting affects overall prevalence. The findings have implications for the use of NPHS data. La traduction du résumé se trouve à la fin de l article. 1. Arthritis Community Research and Evaluation Unit, Toronto Western Research Institute, University Health Network, Toronto, ON 2. Department of Public Health Sciences, University of Toronto, Toronto Correspondence and reprint requests: Anthony V. Perruccio, Arthritis Community Research and Evaluation Unit, Division of Outcomes and Population Health, Toronto Western Research Institute, University Health Network, 399 Bathurst St., MP10-322, Toronto, ON M5T 2S8, Tel: ext. 3166, Fax: , perrucci@uhnres.utoronto.ca Acknowledgements: Supported by a grant from the Ontario Ministry of Health and Long-Term Care through their Health System-Linked Research Unit grant scheme. The responsibility for the use and interpretation of the data is entirely that of the authors. In most countries, population estimates on occurrence of health conditions such as arthritis are collected by population surveys. In the last decade in Canada, the major source of such data have been the 1994, 1996, and 1998 National Population Health Surveys (NPHS). 1 Analysis of data from each cycle of the NPHS indicated that the prevalence of arthritis had increased by almost 3 percentage points over the four years, far exceeding expected increases due to the aging of the population. 2-5 A possible explanation for this increase lies in changes in survey methodology over time. In population surveys, some data inevitably are collected by proxy, either because a designated respondent is unable to respond for themselves, or by design. For clarity, persons who respond for themselves are referred to as self-respondents, while persons who have responses provided for them by others are referred to as proxy respondents. Household members providing the information for others are referred to as proxy responders. Proxy interviews were part of the initial design of the NPHS in order to avoid the high cost and time requirements of obtaining information directly from each household member. As well, proxy reporting was allowed for designated respondents for reasons of illness/incapacity. The use of proxies in health surveys raises issues of whether information obtained by proxy is as accurate as that which is selfreported. In a growing body of literature, lower estimates of hospitalization, activity limitations, emotional problems, pain and chronic conditions have been attributed to proxy reporting The proxy effect on the 1994 and 1996 NPHS has been shown by Shields et al. 6 This effect was most marked for arthritis/rheumatism, back problems and allergies (non-food): proxy responders were almost 2 times less likely to report arthritis than self-responders, 1.5 times less likely for back problems, and 1.6 times less likely for allergies. Following these findings, attempts were made to avoid proxy responses in subsequent administrations of the survey. Consequently, the proportion of proxy respondents decreased from the 1 st to 3 rd administration. It was expected that this change could have had an impact on prevalence estimates for arthritis over time. MAY JUNE 2004 CANADIAN JOURNAL OF PUBLIC HEALTH 169

2 The objectives of this paper were to 1) determine whether changes in the prevalence of arthritis over time can be explained by a) potential changes in the age- and sex-structure of the population or b) the variation in the proportion of proxy respondents, and 2) the extent to which proxy reporting affects the overall estimate of prevalence. METHODS Data source Analyses were based on cross-sectional household data from the 1994/95, 1996/97 and 1998/99 cycles of the NPHS. The target population included all household residents in each Canadian province, excluding populations on Indian reserves, Canadian Forces Bases, and some remote areas. Analyses and results are based on those 15+ years of age. The first part of the interview collected information for every member of participating households from one knowledgeable member. The questions posed sought to establish, among other things, the presence of any chronic conditions. Arthritis or rheumatism was included in a list of health conditions presented in conjunction with the question, Do you have any of the following long-term conditions that have been diagnosed by a health professional? Long-term was defined as having lasted or expected to last six months or longer. The second part of the interview collected additional, more personal, in-depth health information from one randomly selected household member (longitudinal panel member). This information was combined with the information collected in the first part of the interview pertaining to that person. The combined data set is used in this study. In the 3 rd cycle only, interviewers were instructed to collect information directly from the longitudinal panel member for the entire questionnaire. Proxy response was accepted if the selected person was unable to answer for reasons of illness/incapacity. Thus, the proportion of proxy respondents is least for the 3 rd cycle (Table I). Statistical analyses To control for possible changes in the ageand sex-structure of the population, both TABLE I Survey Sample Description Sample size 16,989 70,884 14,682 Response rate 88.7% 95.6% 98.5% Proportion of proxy respondents: Unweighted 27.3% 26.1% 13.2% Weighted 33.2% 25.1% 15.9% TABLE II Crude and Age- and Sex-standardized Prevalence of Arthritis % Prevalence Percent Prevalence (95% CI) Crude Overall 13.4% 14.5% 16.0% ( ) ( ) ( ) Self-respondents 15.8% 16.5% 17.4% ( ) ( ) ( ) Proxy respondents 8.7% 8.5% 8.4% ( ) ( ) ( ) Age-/Sex-standardized Overall 13.8% 14.7% 15.9% ( ) ( ) ( ) Self-respondents 14.4% 15.8% 16.9% ( ) ( ) ( ) Proxy respondents 12.1% 10.9% 9.5% ( ) ( ) ( ) Age Groups Figure 1. Prevalence of arthritis by age groups, NPHS 94/95 & 98/99 * p<0.05 for difference in prevalence between cycles p<0.01 for difference in prevalence between cycles crude and age- and sex-standardized prevalence estimates of arthritis were calculated for each NPHS cycle. Ten-year age groups (15-24 to 75+) were employed for standardization, using 1996 Canadian population figures as the standard population. Prevalence estimates, by 10-year age groups, were compared between 1 st and 3 rd cycles. Analyses were also performed stratified by respondent type (self vs. proxy), with overall crude and age- and sex-standardized * NPHS 98 NPHS 94 * estimates determined for each cycle. As prevalence estimates were found to differ between self-respondents and proxy respondents, a ratio of proxy prevalence to self prevalence was calculated for each age/sex categorization. Applying weighted self-respondent age/sex point-prevalence estimates to the corresponding weighted proxy-respondent sample (e.g., prevalence among self-respondent males aged imposed on proxy-respondent male aged sample), an adjusted overall 170 REVUE CANADIENNE DE SANTÉ PUBLIQUE VOLUME 95, NO. 3

3 TABLE III Estimates of Prevalence and Numbers with Arthritis (aged 15+): Unadjusted and Adjusted* Ratio Canadian prevalence of arthritis was calculated for each cycle. Data were weighted taking into account the sample design, adjustments for nonresponse, and post-stratification, and are representative of the household population aged 15+ years. Variances of estimates (point prevalence and differences between proportions) were calculated employing the bootstrap method as recommended by Statistics Canada, 1 using coordinated bootstrap weights. RESULTS The overall crude prevalence of arthritis increased by 2.6 percentage points (95% CI: , p<0.01) from 13.4% to 16.0% over the 4 years (Table II), an increase of almost 20%. On average, an Unadjusted Adjusted Unadjusted Adjusted Unadjusted Adjusted Total number of self-respondents reporting arthritis 2,383,194 2,383,194 2,896,556 2,896,556 3,479,504 3,479,504 Total number of proxy respondents reporting arthritis 649, , , , , ,033 Total number reporting arthritis 3,032,853 3,169,025 3,399,004 3,653,913 3,797,312 4,040,537 Percent prevalence of arthritis, overall 13% 14% 14% 16% 16% 17% * Adjusted for proxy reporting by imposing age- and sex-specific self-respondent prevalence estimates onto proxy-respondent sample Figure 2. Ratio Overall: 0.48 Male: 0.60 Female: Age Groups male female Ratio of proxy prevalence to self-prevalence, by age & sex; NPHS 98/99 8 percentage-point lower prevalence was found in the proxy-respondent group, compared to self-respondents (Table II). Although the crude prevalence remained stable across cycles for the proxy-respondent group, a 1.6 (95% CI: , p<0.01) percentage-point increase was observed in the self-respondent group. Overall age- and sex-standardized prevalence estimates were not significantly different from crude estimates (Table II). Standardized estimates differed from crude estimates, however, within both proxyand self-respondent groups. While standardized self-respondent estimates increased by 1 percentage point per cycle, standardized proxy-respondent estimates decreased by this amount. The latter is due to a decreasing proportion of younger proxy respondents over time. Comparing point-prevalence estimates of arthritis between 1 st and 3 rd cycles for decades of age, significant increases were observed for the year age groups (Figure 1). An increase of 3.3 (95% CI: , p<0.05) percentage points was observed for the age group, 5.5 (95% CI: , p<0.01) for the age group, and 4.6 (95% CI: , p<0.05) for the age group. The overall ratio of proxy prevalence to self-prevalence for the 3 rd cycle was 0.48, with ratios of 0.60 and 0.48 for males and females, respectively (Figure 2). The disparity between proxy and self-respondents decreased with age from a low of 0.20 in the youngest age group to 0.81 in the oldest age group. When age- and sex-specific pointprevalence estimates of self-respondents were imposed on the corresponding proxyrespondent sample for each cycle, similar results were observed. The overall adjusted prevalence of arthritis increased by at least 1 percentage point within each cycle (Table III). For the 1998/99 cycle, this represents an estimated additional 250,000 persons with the disease. DISCUSSION Possible explanations for the observed increase in arthritis prevalence over time could be changes in the age- and sexstructure of the population or in the proportion of proxy respondents over time, or both. As both crude and age- and sexstandardized prevalence estimates of arthritis increased similarly over the study period, changes in the age- and sex-structure of the population over time were ruled out as contributing factors. A decrease in the proportion of proxy respondents over time in the NPHS also did not appear to account for increases in the prevalence of arthritis. Increases were found to occur only in the self-respondent group. Also, adjustments for proxy responses using self-respondent prevalence estimates had similar effects on the overall prevalence estimate in each wave. To our knowledge, this is the first study that has looked at the effects of changing proportions of proxy respondents over time on changing prevalence estimates in longitudinal surveys. Previous studies have shown that there are inconsistencies in the reporting of MAY JUNE 2004 CANADIAN JOURNAL OF PUBLIC HEALTH 171

4 arthritis in health surveys, comparing either proxy reports to self-reports or household interviews to medical records. 6,13,22-25 The level of agreement between proxy reports and self-reports with respect to health status has been shown to depend on a number of factors, including frequency of contact between proxy reporter and person for whom the information is supplied; 26,27 characteristics of the illness in question, including severity, perceived seriousness, frequency and severity of symptoms and functional limitations; 6,19,22,23,28 and, for general reporting of health status, the social determinants of reporting illness Further, in testing the differences in reporting family status and other characteristics, results have shown that differences occur according to which family member is interviewed. 11,32 Shields 6 found that for both males and females aged <24, the proxy reporter was most often a parent, usually the mother. For those aged >24, proxy reporters were typically spouses. There is some evidence to suggest that male heads of households are particularly poor at reporting symptoms of others in their household. 11 Some studies have suggested that women may be more inclined than men to report health conditions, both for themselves and for others, 20,27,29,33 but in the 1996/97 NPHS, Shields 6 did not find increased odds of reporting arthritis for those whose information was reported by women. Furthermore, studies indicate an under-reporting by proxy reporters of many of the symptoms associated with arthritis, including presence and/or severity of pain, joint troubles, and disability. 16,22 While one study 19 reported that those who have responses provided on their behalf are more likely to be of better health, Shields analysis of NPHS 2 nd cycle data showed that individuals in poor, fair or good health had higher odds of having their information reported by proxy, compared with those in excellent health. 6 Our findings corroborate those of others that prevalence estimates of arthritis from proxy-respondents were significantly lower than estimates from self-respondents, and suggest that arthritis in younger people and in women is more likely to be underreported by proxy responders. It may be that the higher likelihood of having arthritis contributes to increased awareness of arthritis in older people. The differences in prevalence estimates between proxy and self-respondents have implications for the use of survey health data in general, and especially so for those who use the NPHS. How proxy responder characteristics (e.g., sex, age, relationship to household member, etc.) affect the reporting of arthritis for household members, and whether an interaction effect exists with household member characteristics, are unknown and require further research. Our findings further beg the question of whether this phenomenon is specific to arthritis or whether it is true for other chronic conditions as well. The effect of proxy reporting is also likely to affect incidence estimates and may limit the utility of the NPHS for studies of the incidence of arthritis and other chronic conditions. Shields found that individuals who had their information reported by proxy in an earlier cycle and then reported their own information in a subsequent cycle had higher odds of reporting a new case of the condition (OR: 2.4, 95% CI: ), compared with those who had a proxy-respondent status in both cycles. 6 Although under-reporting of arthritis by proxy responders has an impact on overall estimates of prevalence of arthritis, the changing proportion of proxy respondents over time does not completely account for the observed increase in prevalence. Adjustment for the effect of proxy reporting suggests the overall prevalence of selfreported arthritis is underestimated by at least 1 percentage point. The substantial increase over time in self-reported arthritis remains unexplained and requires further investigation. A possible hypothesis is that this may be related to increased awareness of arthritis medication. Over the time period of the study, there was considerable attention to new drugs such as COX2 anti-inflammatory drugs and increased advertising of prescription drugs While direct-to-consumer advertising (DTCA) of prescription medicines is prohibited in Canada, Canadians nevertheless see advertisements in US print media and cable television, where from 1996 to 2000, spending on DTCA more than tripled. 34 The extent to which increased media attention to arthritis drugs might have led to increased reporting of arthritis is unknown. REFERENCES 1. Statistics Canada. National Population Health Survey: 1994/1995, 1996/1997 and 1998/1999 master files. Ottawa: Statistics Canada, 1995, 1997 and World Bank. World Development Report 1993: Investing in Health. New York, NY: Oxford University Press, Global Aging: Comparative Indicators and Future Trends. Washington, DC: US Department of Commerce. Economics and Statistics Administration. Bureau of Census, September Badley EM, Crotty M. An international comparison of the estimated effect of the aging of the population on the major cause of disablement, musculoskeletal disorders. J Rheumatol 1995;22: Badley E, Wang P. Arthritis and the aging population: Projections of arthritis prevalence in Canada 1991 to J Rheumatol 1998;25: Shields M. Proxy reporting in the National Population Health Survey. Health Reports 2000;12(1): Anderson R, Kasper J, Frankel MR, Banks MJ, Daughety VS. Total Survey Error. San Francisco, CA: Jossey-Bass, Cannell C, Fowler F. A study of the reporting of visits to doctors in the National Health Survey. Ann Arbor, MI: University of Michigan, Cannell CF, Marquis KH, Laurent A. A summary of studies of interviewing methodology. Vital and Health Statistics 1977;2(69): Cartwright A. Memory errors in a morbidity survey. Milbank Mem Fund Q 1963;41: Cartwright A. The effect of obtaining information from different informants on a family morbidity inquiry. Applied Statistics 1957;6(1): Tompkins L, Massey JT. Using a most knowledge respondent rule in a household telephone survey. American Statistical Association: Proceedings of the Section on Survey Research Methods. Washington, DC: American Statistical Association, 1986; Clarridge BR, Massagli MP. The use of female spouse proxies in common symptom reporting. Medical Care 1989;27(4): Miller RE, Massagli MP, Clarridge BR. Quality of proxy vs. self reports: Evidence from a health survey with repeated measures. American Statistical Association: Proceedings of the Section on Survey Research Methods. Washington, DC: American Statistical Association, 1986; National Center for Health Statistics. Health interview responses compared with medical records. Vital and Health Statistics 1965;2(7): Grootendorst PV, Feeny DJ, Furlong W. Does it matter whom and how you ask? Inter- and intrarater agreement in the Ontario Health Survey. J Clin Epidemiol 1997;50(2): Kovar MG, Wright RA. An experiment with alternate respondent rules in the National Health Interview Survey. American Statistical Association: Proceedings of the Section on Survey Research Methods. Washington, DC: American Statistical Association, 1973; Grootendorst P. Results of an investigation into the integrity of the Ontario Health Survey. Centre for Health Economics and Policy Analysis: Working Paper Series McMaster University, Mosely R II, Wolinsky F. The use of proxies in health surveys: Substantive and policy implications. Medical Care 1986;24(6): Madow WC. Net differences in interview data on chronic conditions and information derived from medical records. Vital and Health Statistics 1973;2(57): REVUE CANADIENNE DE SANTÉ PUBLIQUE VOLUME 95, NO. 3

5 21. Heliovaara M, Aromaa A, Klaukka T, Knekt P, Joukamaa M, Impivaara O. Reliability and validity of interview data on chronic diseases: The Mini-Finland Health Survey. J Clin Epidemiol 1993;46(2): Tennant A, Badley EM, Sullivan M. Investigating the proxy effect and the saliency principle in household based postal questionnaires. J Epidemiol Community Health 1991;45: Beckett M, Weinstein M, Goldman N, Yu- Hsuan L. Do health interview surveys yield reliable data on chronic illness among older respondents? Am J Epidemiol 2000;151(3): Haapanen N, Miilunpalo S, Pasanen M, Oja P, Vuori I. Agreement between questionnaire data and medical records of chronic diseases in middle-aged and elderly Finnish men and women. Am J Epidemiol 1997;145: Edwards WS, Winn DM, Kurlantzick V, Sheridan S, Berk ML, Retchin S, et al. Evaluation of National Health Interview Survey Diagnostic Reporting. National Center for Health Statistics. Vital Health Stat 1994;2(120). 26. The Medical Research Council Cognitive Function and Ageing Study. Survey into the health problems of elderly people: A comparison of self-report with proxy information. Int J Epidemiol 2000;29: The Medical Research Council Cognitive Function and Ageing Study. Survey into the health problems of elderly people: Multivariate analysis of concordance between self-report and proxy information. Int J Epidemiol 2000;29: Mechanic D. Sociological dimensions of illness behavior. Soc Sci Med 1995;41(9): Marcus AC, Seeman TE. Sex differences in reports of illness and disability: A preliminary test of the fixed role obligations hypothesis. J Health Soc Behav 1981;22: Mechanic D. The concept of illness behaviour: Culture, situation and personal predisposition. Psychological Med 1986;16: Radley A, Billig M. Accounts of health and illness: Dilemmas and representations. Sociology of Health and Illness 1996;18(2): Ferber R. On the reliability of responses secured in sample survey. J Am Statistical Assoc 1955;50: Nathanson CA. Sex roles as variables in the interpretation of morbidity data: A methodological critique. Int J Epidemiol 1978;7: Rosenthal MB, Berndt ER, Donohue JM, Frank RG, Epstein AM. Promotion of prescription drugs to consumers. New Engl J Med 2002;346(7): RÉSUMÉ 35. Harvard Health Letters. Should you be taking a COX-2 inhibitor? 2001;27(1): Kasman N, Badley EM. Arthritis-related prescription medications. In: Badley EM, DesMeules M (Eds.), Arthritis in Canada: An Ongoing Challenge. Ottawa: Health Canada, Received: April 4, 2003 Accepted: December 2, 2003 Contexte : L analyse des Enquêtes nationales sur la santé de la population (ENSP) de à fait état d une augmentation significative et plus importante que prévu, soit de 13,4 % à 16 %, du taux d arthrite déclaré dans la population à domicile de 15 ans et plus. Notre étude visait à déterminer si la prévalence croissante de l arthrite peut s expliquer a) par les changements dans la structure par âge et par sexe de la population ou b) par la variation de la proportion d enquêtéssubstituts et l effet possible des déclarations par personne interposée sur l estimation globale de la prévalence. Méthode : Analyses globales de l arthrite ou du rhumatisme déclarés dans les cycles , et de l ENSP à titre de problèmes de santé durables diagnostiqués par un professionnel de la santé. Analyses stratifiées par type de répondant pour expliquer la proportion décroissante des enquêtés-substituts au fil du temps (de 33 % à 16 %). Résultats : Les estimations globales sur la prévalence, normalisées selon l âge et le sexe, étaient semblables aux estimations non rectifiées. La prévalence non rectifiée de l arthrite déclarée par les enquêtés-substituts est restée stable (à environ 8,5 %), tandis que chez les répondants directs, elle est passée de 15,8 % à 17,4 % au fil des trois cycles. Après ajustement pour la plus faible prévalence déclarée par les enquêtés-substituts, la prévalence estimative globale de l arthrite a gagné au moins un point de pourcentage par année de cycle. La disparité entre les déclarations directes et par personnes interposées était plus prononcée chez les jeunes et chez les femmes. Conclusion : Il existe une disparité significative entre les répondants directs et les enquêtéssubstituts dans les estimations sur la prévalence selon l âge et le sexe. L accroissement de la prévalence de l arthrite au fil du temps résulte de sa déclaration accrue par les répondants directs. Les réponses par personnes interposées ont un effet sur la prévalence globale. Ces constatations ont des conséquences pour l utilisation des données des ENSP. MAY JUNE 2004 CANADIAN JOURNAL OF PUBLIC HEALTH 173