Response Rates to a Questionnaire 26 Years after Baseline Examination with Minimal Interim Participant Contact and Baseline Differences between Respondents and Nonrespondents
Amber Pirzada,
Lijing L. Yan,
Daniel B. Garside,
Linda Schiffer,
Alan R. Dyer and
Martha L. Daviglus
From the Department of Preventive Medicine, Feinberg School of Medicine, Northwestern University, Chicago, IL.
Received for publication April 2, 2003; accepted for publication July 18, 2003.
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ABSTRACT
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Research on response rates to surveys mailed to study participants decades after baseline examination, with minimal interim contact, is limited. This paper documents response rates to a 26-year follow-up survey of surviving participants from a large cohort in Illinois and compares baseline characteristics of nonrespondents and respondents. Mortality follow-up of the Chicago Heart Association Detection Project in Industry 19671973 cohort involved minimal or no participant contact since baseline. In 1996, a 26-year follow-up questionnaire was mailed to all surviving participants aged 65 years or older. Current addresses were obtained from the Health Care Financing Administration for 96.5 percent of 12,409 participants in our analyses. Total response rates were 59.8 percent and, for participants for whom Health Care Financing Administration addresses were available, 60.8 percent. A higher response rate was obtained for younger recipients, men, Whites, more-educated persons, nonsmokers, and those with a better cardiovascular risk profile at baseline. A graded negative relation was found between number of cardiovascular risk factors at baseline and response rates obtained in 1996. Use of Health Care Financing Administration records as an additional follow-up method and factors that influence response rates are discussed. In conclusion, long-term follow-up of older surviving participants is feasible if current addresses can be obtained from standardized sources.
aged; data collection; epidemiologic methods; follow-up studies; health surveys
Abbreviations:
Abbreviations: HCFA, Health Care Financing Administration; NCOA, National Change of Address.
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INTRODUCTION
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Longitudinal studies play an important role in epidemiologic research, and information on initial participation and retention rates is essential to assess potential biases related to nonresponse and loss to follow-up (1, 2). Consequently, response rates and baseline comparisons between respondents and nonrespondents are routinely reported in published cohort studies (38). However, most prospective studies involve either only mortality follow-up (9, 10) or repeated examinations or contact with participants at regular intervals (1113). Little information is available on response rates for recontacting surviving participants after an extended period postbaseline (14, 15). In addition, although cross-sectional differences in demographic and health characteristics between participants and nonparticipants have been documented (1622), little is known about baseline differences in major cardiovascular risk factors between respondents and nonrespondents to surveys mailed decades after initial screening, with minimal interim contact with participants.
This paper documents the results of a 26-year follow-up postal survey of surviving men and women aged 65 years or older from a large Illinois cohortthe Chicago Heart Association Detection Project in Industrywith minimal or no contact after baseline examination. Current addresses were provided by the Health Care Financing Administration (HCFA), now called the Centers for Medicare & Medicaid Services. We report response rates to a health questionnaire and compare baseline characteristics and cardiovascular risk profiles between respondents and nonrespondents.
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MATERIALS AND METHODS
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Baseline examination
Methods of the Chicago Heart Association Detection Project in Industry study have been described previously (23, 24). Briefly, 39,522 young adult, middle-aged, and older men and women from different racial groups were screened from November 1967 through January 1973. All employees of 84 cooperating Chicago-area organizations, about 75,000 people, were invited to participate. Examinations were performed by trained field teams following standardized protocols. Information collected included demographic and socioeconomic characteristics and cardiovascular risk factor measurements. Prior to 1979, vital status was ascertained by means of direct mail, telephone, contacts with employers, and matching of study records with Social Security Administration files. Since 1979, follow-up has been conducted by researchers at Northwestern University (Chicago), without further participant contact, by matching study records with the National Death Index and HCFA billing records.
Follow-up survey at 26 years postbaseline
Of the 20,912 participants who would have been aged 65 years or older in 1996, 7,511 were known to be deceased by October 1, 1996. In 1996, a 26-year follow-up questionnaire was mailed to all surviving study participants aged 65 years or older (n = 13,401). For 92.2 percent of them, current addresses were obtained from HCFA by matching records with the full name, sex, date of birth, and social security number obtained at baseline. For the remaining 1,040 participants who were not matched in the HCFA database, their last known address (prior to 1979) was used. As mandated by HCFA, a letter was first mailed to all participants informing them about the forthcoming questionnaire and emphasizing the voluntary nature of the study. Surveys returned by the US Postal Service with forwarding addresses were remailed to the new address. The four-page questionnaire, sent via nonprofit bulk mail, included assessments of risk factors, health-related quality of life, habitual exercise pattern, lifestyle, medical history, and treatment of major diseases. Questionnaires were individualized with a letter from the principal investigator on high-quality university stationery, and business-reply return envelopes were supplied. A toll-free number was provided for participants needing assistance or wishing to complete the questionnaire over the telephone. The study protocol was approved by Northwestern Universitys Institutional Review Board.
Of the 13,401 participants to whom surveys were mailed, 159 (1.2 percent) were reported by their relatives to be deceased, and 833 (6.2 percent) questionnaires were returned by the US Postal Service without forwarding addresses. These questionnaires were excluded; the remaining 12,409 persons (6,263 men and 6,146 women) constitute the denominator for computing response rates. Only 526 (4.2 percent) persons explicitly refused to participate, and they were included in the sample as nonrespondents. Average length of follow-up from baseline examination to receipt of a completed survey was 26 years.
Data analyses
In this paper, we report response rates by baseline characteristics and by address source (HCFA or original addresses), and baseline differences between respondents and nonrespondents. We also report the response rate obtained if the 833 questionnaires returned without forwarding addresses are included in the denominator. Response is defined as returning a filled-out questionnaire. The significance of between-group differences was tested by using two-tailed t tests for continuous variables and
2 tests for categorical variables. Blood pressure, serum cholesterol, and body mass index (weight in kilograms divided by square of height in meters) were classified according to current standards (2527). Baseline cardiovascular risk status was categorized by using the following six risk factors: history of diabetes, history of myocardial infarction, any electrocardiographic abnormalities, currently smoking, systolic blood pressure
140 mmHg or diastolic blood pressure
90 mmHg or use of antihypertensive medication, and serum total cholesterol level
240 mg/dl. Participants were classified as having zero, any one, any two, or three or more risk factors. Persons without any risk factors who had optimal blood pressure (systolic blood pressure/diastolic blood pressure
120/80 mmHg) and cholesterol (<200 mg/dl) levels were classified separately as the "low-risk" group (10, 28).
To predict the probability of nonresponse by baseline characteristics and risk factors, we conducted multivariable logistic regression with nonresponse (yes/no) as the outcome variable and age, sex, education, smoking, blood pressure, cholesterol, and body mass index as the explanatory variables. All analyses were performed with SAS software, version 8.02 (SAS Institute, Inc., Cary, North Carolina).
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RESULTS
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Response rates
Among the 12,409 participants aged 36 years or older at baseline (mean age in 1996, 74.2 (standard deviation, 6.3) years), the total response rate was 59.8 percent. Item nonresponse rates in returned questionnaires were low (<2.6 percent). Updated addresses from HCFA were available for 96.5 percent (n = 11,979) of the 12,409 persons in the main analyses, and response rates were significantly higher for participants for whom HCFA addresses were available (60.8 percent) compared with those for whom last known addresses were used (30.9 percent). If participants whose questionnaires were returned without forwarding addresses (n = 833) were included in the denominator, the total response rate was 56.0 percent. Surveys with HCFA addresses constituted 33.3 percent of the 833 questionnaires returned without forwarding addresses (2.3 percent of all HCFA addresses).
Response rates were higher for younger recipients, men, Whites, more-educated persons, nonsmokers, and persons with better health profiles as indicated by blood pressure, cholesterol, and body mass index (table 1). A graded inverse relation was found between baseline cardiovascular risk status and response rate. The response rate for low-risk persons was 70.1 percent compared with 66.6 percent, 60.2 percent, 55.7 percent, and 51.0 percent for persons with zero, any one, any two, or three or more risk factors at baseline, respectively.
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TABLE 1. Response rates (%), total and by baseline characteristics, to a 26-year follow-up postal questionnaire in 1996, Chicago Heart Association Detection Project in Industry, 19671973
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Baseline characteristics of respondents and nonrespondents
At baseline, mean age of the 12,409 participants was 48.5 years, 23.7 percent did not have a high school education, and 32.1 percent were smokers (table 2). On average, they had above optimal mean levels of systolic blood pressure (136.8 mmHg) and diastolic blood pressure (81.2 mmHg), cholesterol (213.9 mg/dl), and body mass index (26.0 kg/m2).
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TABLE 2. Differences in baseline characteristics (19671973) between respondents and nonrespondents, Chicago Heart Association Detection Project in Industry 26-year follow-up study, 1996
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Differential patterns were apparent when distributions of various baseline characteristics were compared among respondents with those among nonrespondents. For example, among respondents, 26.8 percent had a college education or higher, whereas 15.8 percent had not completed high school; these figures compare with 10.3 percent and 35.4 percent of nonrespondents, respectively (table 2). Compared with respondents, higher percentages of nonrespondents were obese, smoked, and had high serum cholesterol and blood pressure levels. All differences between groups were statistically significant at the p = 0.001 level. Results were similar after adjustment for age and sex (data not shown).
Baseline predictors of response
Table 3 shows multivariable-adjusted odds ratios for nonresponse to the 26-year follow-up survey for the total sample and for men and women separately. Consistent with patterns reported in table 2, female sex, non-White race, older age, lower educational level, cigarette smoking, overweight, and obesity were significant predictors of nonresponse. For example, nonresponse was more than four times as likely among participants with less than a high school degree than those with a college degree or higher. Higher baseline blood pressure level was a significant predictor of nonresponse among women but not men. In these multivariable models, cholesterol was not a significant predictor of response at the p = 0.05 level.
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TABLE 3. Multivariable-adjusted* odds ratios of not responding to the 26-year follow-up questionnaires, Chicago Heart Association Detection Project in Industry, 1996
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DISCUSSION
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Although there is much documentation on response rates for initial study recruitment or continued follow-up (38, 2931), this paper provides 26-year response rates for surviving participants aged 65 years or older with minimal contact since baseline. Among the 12,409 participants, the overall response rate was 59.8 percent. Current addresses from HCFA were available for 96.5 percent of the participants, and their response rate was about twice as high compared with those for whom only original addresses were available. In both univariable and multivariable analyses, higher response rates were also found for younger recipients, men, Whites, more-educated persons, and those with a better baseline cardiovascular risk profile. A significant graded relation was found between response rates and baseline cardiovascular risk status established about 26 years ago.
Few reports are available on the feasibility of conducting follow-up surveys decades after last contact. Clarke et al. (14) reported results of a pilot 25-year follow-up resurvey in the Whitehall study of London Civil Servants. In 1995, a questionnaire was mailed to a random sample of 401 men aged 65 years or older (4 percent of surviving participants) whose current addresses were obtained from health authorities in the United Kingdom. The response rate after the initial mailing was 55 percent. A further 12 percent and 6 percent of men responded after the first and second reminder letters, respectively (14). Since we sent no reminder letters, our overall response rate (59.8 percent) is similar. However, the response rate for older men in our sample was 65.9 percent, about 11 percent higher than their initial response rate.
Cross-sectional or longitudinal studies of older persons that used more-aggressive recruitment strategies have obtained higher response rates than ours (15, 22, 31). In a recent study evaluating a protocol to contact participants after 2527 years, about 72.8 percent of the 585 participants not known to be deceased responded to a mailed questionnaire (15) (authors computations based on figure 1 of Weinberger et al. (15)). However, a $15 incentive was provided for completing the questionnaire, which, along with other strategies used (including hospital chart audits and telephone follow-up of nonrespondents), is not feasible for follow-up of very large cohorts such as ours.
Access to the comprehensive national HCFA address database enabled us to obtain current addresses for the majority of surviving participants 26 years after baseline, although many had migrated out of the Chicago area. Medicare data have previously been used in health economic and epidemiologic research (32). Our approach of linking baseline records with HCFA data to obtain current addresses of surviving participants aged 65 years or older provides an additional method for long-term follow-up. However, Medicare enrollment is not universal among those eligible, and differentials across race, sex, and socioeconomic status have been documented (33, 34). According to a 1996 report, approximately 97 percent of the elderly population was enrolled in Medicare (35). Our results are likely to be influenced by such differentials as well as by the inaccuracy of a small proportion of HCFA addresses (2.3 percent of HCFA addresses were considered undeliverable by the US Postal Service).
Other sources of updated address information include US Postal Service computerized address updating systems (National Change of Address (NCOA) system and FASTforward), credit bureaus, commercial tracing companies, and local and state sources. Although these sources may be more accessible to researchers, they may be limited in the accuracy of matches, completeness of data, or extent of geographic coverage (3638). In the NCOA system, persons are matched only by names and last-known addresses, raising concerns about accuracy, especially since people may have changed their last or even first names. The US Postal Service estimates that 2550 percent of address changes are not attained by NCOA because of inaccurate matches, change of addresses not filed with the US Postal Service, and the time lag from filing of an address change to its availability in the NCOA system. Additionally, address changes are maintained in the NCOA file for only 3 years (37, 38), making it impossible to track participants who have not been contacted for an extended period. For example, the list of all Chicago Heart Association Detection Project in Industry participants was first submitted to a company providing updated contact information to researchers, which used NCOA and other databases in its searches; however, updated addresses were obtained for only about 2 percent of participants.
The new Health Insurance Portability and Accountability Act (HIPAA) privacy regulations that took effect in April 2003 will no doubt impact the ability of researchers to gain access to such information from the Centers for Medicare & Medicaid Services or other national sources. In some cases, it may be possible and easier to obtain updated contact information from other sources such as NCOA or FASTforward. However, follow-up studies such as ourswhich depend on accurately linking baseline and follow-up data and in which there was no participant contact for an extended periodcannot be conducted without access to updated address information from standardized national sources. In such cases, the institutional review board or privacy board may grant a waiver of HIPAA authorization if the research cannot be performed without a waiver or access to personal health information and if it involves no more than minimal risk to participants privacy (3941).
Mailing strategies that improve response rates to postal surveys have been identified (42, 43). The brevity of our questionnaire, the individualized high-quality materials used, and the availability of a toll-free telephone number facilitated acceptance and completion of our survey. However, our response rates could have been higher if our budget had enabled us to mail reminder letters to nonrespondents (14), use certified or first class mail (43), or provide incentives (44, 45). Additionally, response rates may have been influenced by the inability of some subjects to recall participating in the baseline examination. Although we could not assess the percentage of nonparticipation due to this reason, this problem was fairly evident in the telephone calls received and in the small number of explicit refusals.
Our study has limitations. First, the response rate was constrained by a limited budget that did not allow use of more-aggressive mailing strategies. Second, HCFA addresses are available only for persons aged 65 years or older, some disabled persons under age 65 years, and those with end-stage renal disease. Hence, this method is not applicable to younger cohorts. Finally, current addresses were not available for a small proportion of our participants aged 65 years or older.
Long-term follow-up of study survivors provides valuable information on health outcomes other than mortality, such as morbidity, functional status, and quality of life. The difficulties associated with complete long-term follow-up have been documented, especially when participants have not been contacted for decades (14). Our study demonstrates that follow-up of older cohort survivors 26 years after baseline is feasible when current addresses can be obtained from comprehensive national sources. Our results also show that survivors with better baseline cardiovascular risk profiles had higher response rates after 26 years than did those with poorer risk profiles. These findings should be considered when conducting follow-up postal surveys of surviving participants of long-term prospective epidemiologic studies.
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ACKNOWLEDGMENTS
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Supported by the American Heart Association and its Chicago and Illinois affiliates; the Illinois Regional Medical Program; the National Heart, Lung, and Blood Institute (HL21010 and HL03387); the Chicago Health Research Foundation; and private donors.
The work of the Chicago Heart Association Detection Project in Industry study was accomplished thanks to the invaluable cooperation of 84 Chicago-area companies and organizations and their officers, staff, and employees, whose volunteer efforts made the project possible. Acknowledgement is also gratefully extended to all Chicago Heart Association staff and volunteers serving the project. Many of these persons are cited by name in Stamler et al. (24).
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NOTES
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Correspondence to Dr. Martha L. Daviglus, Department of Preventive Medicine, Feinberg School of Medicine, Northwestern University, 680 North Lake Shore Drive, Suite 1102, Chicago, IL 60611-4402 (e-mail: daviglus{at}northwestern.edu). 
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