1 Fred Hutchinson Cancer Research Center, Seattle, WA.
2 Department of Epidemiology, School of Public Health and Community Medicine, University of Washington, Seattle, WA.
Received for publication January 28, 2002; accepted for publication August 13, 2002.
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ABSTRACT |
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bias (epidemiology); case-control studies; data collection; epidemiologic methods; selection bias; telephone
Abbreviations: Abbreviation: RDD, random digit dialing.
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INTRODUCTION |
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Several studies have compared respondents who refuse to participate, or those who initially refuse and later agree, with respondents who readily agree to participate. Most have found that reluctant or difficult-to-reach respondents are older and less educated than respondents who readily agree (49). Differences with respect to income, occupation, race, and marital status have been inconsistent (4, 5, 7, 912). Three studies have compared reproductive or lifestyle characteristics of initial refusers with those of initial responders (3, 7, 11). Three studies have found that while characteristics of initially refusing respondents differed from those of other respondents, the inclusion of those persons data had little overall effect on the results (4, 7, 11). In addition, one study found that data from initial refusers were less useful because these persons refused to answer more questions than other respondents (9). Since recruiting reluctant respondents is expensive and lengthens the time required to complete interviews, it is important to determine whether or not this additional expense is warranted.
We undertook this study to further address the question of differences in demographic, reproductive, health, and lifestyle characteristics between control respondents according to the level of effort required to recruit them.
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MATERIALS AND METHODS |
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Controls were selected using a one-step stratification design (20). Controls were selected so that each 5-year age group would have approximately the same number of controls as did the cases for that particular study. Eligible controls were asked whether they would be willing to receive a letter describing the study. If they were willing, a letter describing the study was mailed within 2 weeks of initial contact. A trained interviewer contacted the potential control after the letter was received to answer questions and to schedule an in-person interview. All phone numbers at which the respondent refused to answer the screening questions for determination of eligibility or, if screened and eligible, refused to receive a letter about the study were recontacted by a different RDD interviewer 36 months after the original phone call. In addition, most phone numbers at which all nine calls were answered by a machine were recontacted at a later date. The recontact included a question as to whether the respondent had had that phone number at the time of the original phone call. Approximately half of the respondents who originally either refused screening or (if eligible after screening) refused to receive a letter agreed on the recontact call. Screening was completed for almost half of the households in which all nine original phone calls had been answered by machine. Interviews were conducted in person by trained interviewers at a place and time most convenient to the respondent. The interviews lasted about an hour. Respondents for the anogenital and oral cancer studies were paid $10 for participation. Respondents for the other studies were not paid.
All respondents were assigned a "reference" year that approximated the distribution of diagnosis years in the corresponding case group. Each control was also assigned a randomly selected "reference" month. Only events that occurred prior to this reference date were recorded during the interview.
We categorized the respondents recruited by RDD into early responders (those who were interviewed within 1 month of the date the letter describing the study was mailed (n = 2,747)), intermediate responders (those who were interviewed 26 months after initial letter contact (n = 837)), late responders (those who were interviewed more than 6 months after initial letter contact (n = 444)), and initial refusers (those who initially refused to participate but agreed after recontact (n = 376)). All respondents who initially refused were grouped together regardless of how much time had elapsed between initial letter and interview. We excluded those who were interviewed 12 months after initial letter contact (n = 1,212), because some of those interviews might have been delayed as a result of interviewer schedules rather than respondent delay. Our categorization was motivated by a common assumption, described by Lin and Schaeffer (21), that the characteristics of people who are difficult to interview become more like those of refusers as the difficulty of interviewing them increases. Table 2 shows the distribution of responses by study.
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All proportions for the intermediate responders, late responders, and initial refusers were standardized to the age distribution of the early responders so that the proportions of respondents with each characteristic could be compared with each other without distortion from the different age structures of the four groups. All variables were included simultaneously in polytomous logistic regression models with the early responders designated the reference group, using Stata statistical software (22). After adjustment for all other variables analyzed, p values were computed for individual variables using Walds test. Differences were considered statistically significant if the two-sided p value was less than 0.05. Results of likelihood ratio tests for inclusion of each variable in models adjusted only for age were similar to the Walds test statistics. Additional significance tests were performed using Bonferronis correction to account for multiple comparisons (23). Results of both tests are presented in the tables.
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RESULTS |
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Women who were intermediate or late responders were similar to early responders with respect to number of pregnancies and oral contraceptive use but were less likely to have ever been tested for infertility. Late responders were less likely to have ever had an induced abortion and to have used noncontraceptive hormones than early responders, but the differences were not statistically significant.
Intermediate- and late-responding women had similar histories of hypertension, diabetes, and cancer and similar body mass indexes.
Late-responding women were more likely than early responders to be current smokers, whereas the smoking history of intermediate responders was similar to that of early responders. Ever use of alcoholic beverages and recent exercise were similar in all three groups.
Comparison of initial refusers with early responders
Initial refusers were older and were more likely to be non-White than early responders. They were less likely to be currently married, to have attended college, and to be employed in a managerial or professional job. They were also less likely than early responders to have a family history of cancer and to use noncontraceptive hormones, but these differences were not statistically significant.
Early responders and initial refusers were similar with respect to all reproductive characteristics except lifetime number of sexual partners. Initial refusers had fewer lifetime sexual partners than early responders.
Initial refusers and early responders had similar health histories, with the exception of diabetes mellitus. Initial refusers were more likely to report a history of diabetes.
Intermediate responders, late responders, and initial refusers were similar to each other and different from early responders with respect to education, non-White race, and history of infertility testing. In addition, late responders were similar to initial refusers with respect to marital status, number of sexual partners, and use of noncontraceptive hormones.
Medical record data
There were no significant differences in medical record data between women who were interviewed and those who refused to be interviewed in the Group Health Cooperative population. Refusers had a higher body mass index than those who were interviewed, but the difference was not statistically significant (table 4).
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DISCUSSION |
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We found only one other study that compared respondents who agreed to be interviewed but delayed the interview with respondents who were interviewed soon after contact (24). Robins (24) conducted in-person interviews of former clients of a child guidance clinic and found that the only difference between early and late responders was proximity to the study site. In contrast, we found several differences between late and early responders and/or intermediate and early responders. Late responders may be as willing to be interviewed as early responders but be less accessible than early respondents because of lifestyle factors and younger age.
Our finding that initial refusers were older, less educated, and less likely to be employed in professional or managerial occupations than early responders is in general agreement with most (4, 7, 11, 12, 24, 25) but not all (25) prior studies. One other study concurred with our results that respondents who initially refused were more likely to be non-White than early responders (9), whereas three others did not (4, 5, 12). Our finding that respondents who initially refused were less likely to be currently married than early responders is in contrast to the studies of both Fitzgerald and Fuller (5) and Kristal et al. (7).
The only other similar study that compared reproductive characteristics differed from our study in that only women of reproductive age who were at risk of pregnancy were included (11). Our findings are consistent with this study with respect to lifetime number of sexual partners but not gravidity or history of induced abortion. Our results did not change when we examined reproductive characteristics only among women of reproductive age.
Our finding that histories of hypertension and cancer were similar for early responders and initial refusers but that initial refusers were more likely to report a history of diabetes is consistent with another comparison of female participants and refusers (3).
Kristal et al. (7) found that initial female refusers were less likely to use alcohol and to smoke cigarettes than those who initially agreed, whereas Criqui et al. (3) found that female refusers were more likely to be current smokers than study participants in a study of heart disease. Those results are in contrast with our findings of no differences with respect to alcohol and cigarette use.
Since all of the studies except one were related to cancer, it is not surprising that early responders were much more likely to have a first- or second-degree relative who had been diagnosed with cancer than either intermediate responders, late responders, or initial refusers. This supports Massey et al.s (26) observation that interest in the study topic improves response. A similar result was found in a study of risk factors for cardiovascular disease (3). In that study, female participants were more likely to have a family history of cardiovascular disease than nonparticipants (3). Although a less-biased group of participants may be recruited if potential respondents are not aware of the specific disease under study, response may be slower.
Although our comparison of medical record data between members of a health maintenance organization who agreed to be interviewed and members who refused to be interviewed revealed no significant differences, the small number of respondents in this group limited our ability to find differences.
Some of the differences between our study and other studies that have compared respondents who initially agreed to participate with those who initially refused may be related to the type of interview administered. Most other studies used telephone interviews or mailed questionnaires, whereas ours utilized in-person interviews. Another reason may be geographic variation or differences over time. Only three of the prior studies were conducted during the 1990s (4, 7, 9); the remainder were conducted prior to 1990. Two of the prior studies that compared respondents who initially agreed to participate with those who initially refused were carried out in the same general geographic area as our study (7, 11). Another difference between our study and prior studies is that we separated respondents who initially agreed into early and late responders, whereas most other studies combined these respondents and compared the combined group with persons who had initially refused.
We found several significant differences when intermediate responders, late responders, and initial refusers were compared with early responders. However, no clear pattern of characteristics that could be useful in generalizing these differences emerged. The only variables that showed a consistent gradient according to difficulty of recruitment were education and race. Age differences between late and early responders and initial refusers and early responders pointed in opposite directions. These findings argue against the common assumption that the characteristics of respondents become more like those of refusers as the difficulty of recruiting them increases. Lin and Schaeffer (21) and Fitzgerald and Fuller (5) reached similar conclusions.
To reduce recall bias, epidemiologic case-control studies which examine events that occurred before the diagnosis date in cases or a similar "reference" date in controls often exclude respondents who are not interviewed within a specified time period after the diagnosis/reference date. In the current study, 24.4 percent of the initial female respondents completed the interview 2 or more months after the letter describing the study was mailed (which was generally within 2 weeks of the RDD contact), and 8.5 percent completed the interview more than 6 months after original contact. Since the initial refusers were recontacted 36 months after the original RDD contact, all of them were interviewed 3 or more months after original contact. The reduction of recall bias must be weighed against the increase in response bias. Weighting factors that are thought to be associated with nonresponse would not adequately compensate for the loss of late responders or initial refusers, since the characteristics of these two groups appear to be different.
Initial refusers comprised only 6.7 percent of our total sample, so excluding them from any of the studies would generally have little effect on the overall distribution of the control group by the characteristics examined. Three studies have shown this to be the case (4, 7, 11). However, analyses by subgroups could be distorted, particularly if the analyses involved two characteristics that were differentially distributed by response. This potential for bias will increase if study participants become more difficult to recruit in the future and an effort is not made to include reluctant respondents (1, 3).
Characteristics of reluctant respondents may well vary over time and by geographic area, so it is difficult to quantify the potential bias resulting from exclusion of inaccessible or less-willing participants. Expending the additional effort required to convert refusers into participants and to delay interviews rather than accept refusal remains the best defense against the creation of unrepresentative or biased samples.
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ACKNOWLEDGMENTS |
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NOTES |
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REFERENCES |
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