1 Slone Epidemiology Unit, Boston University School of Medicine, Brookline, MA.
2 Howard University Cancer Center, Howard University College of Medicine, Washington, DC.
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ABSTRACT |
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blacks; epidemiologic methods; follow-up studies; women
Abbreviations: BWHS, Black Women's Health Study; NCOA, National Change of Address.
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INTRODUCTION |
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A high retention rate is important for obtaining valid results in follow-up studies because the possibility of bias resulting from selective losses due to nonresponse increases as the proportion of subjects retained in the study decreases. There is a growing body of literature on methods to enhance retention of participants in follow-up studies. Most of the literature is based on studies of White men and women (24
). In a study that included both Black and White Americans, attrition was higher among the Black Americans (5
). US Census Bureau statistics indicate that Black Americans move more frequently than do White Americans and that younger persons move more often than do older persons (6
). The BWHS follows a relatively young cohort with a high rate of moving. The first biennial follow-up has been completed. In this report, we describe methods used to follow participants; assess the relation of moving to the participation rate; and compare the characteristics of respondents, nonrespondents, and lost participants.
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MATERIALS AND METHODS |
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Follow-up questionnaires
The participants are followed at 2-year intervals through postal questionnaires. Up to six questionnaires are sent to nonrespondents at intervals of 23 months. After five mailings, telephone calls are made to participants who have not responded. This report is based on data collected through the end of the 1997 follow-up.
Newsletters
The original plan was to send newsletters to participants once a year to update them on study news and results. However, when the first newsletter was sent in 1996, many were returned as undeliverable by the postal service; most post offices forward mail for only 6 months, and many of the addresses had changed more than 6 months previously. Since that time, professionally produced newsletters have been sent every 6 months. The newsletter updates participants on the status of the study, emphasizes the importance of their continued participation, answers frequently asked questions, and profiles the advisory board members and staff. Photographs of the staff have been included to help personalize the study. The newsletter contains a detachable, postage-paid postcard that participants can return with name, address, and telephone number changes and with e-mail addresses.
Toll-free telephone number
The BWHS has a toll-free telephone number that participants can call. Some have called to report address changes.
Questionnaire information used for locating participants
On the baseline questionnaire, 58.2 percent of participants provided the name and address of a contact. The corresponding number for the 1997 follow-up questionnaire was 65.0 percent. On the baseline questionnaire, 78.4 percent of participants also provided a Social Security number.
Sources of information on address changes and vital status of participants
Address changes are obtained from the postal service, National Change of Address (NCOA) files, telephone directories, commercial search firms (e.g., credit bureaus), contacts, friends, family members, and the participants themselves. NCOA files are maintained by the post office and are available through licensed vendors. They provide information on changes of address that have occurred in the previous 36 months. Sources of information on vital status include the National Death Index, the Social Security Death Master File, the postal service, contacts, friends, and relatives.
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RESULTS |
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Between March 1995 and December 1998, there were 56,842 address changes. Table 1 shows the sources of information for address changes. The major source of information was the participants themselves: 31,153 changes were recorded on the follow-up questionnaires or on postcards from newsletters that had been forwarded to participants by the postal service. We also learned of 13,152 changes through NCOA files and of 4,026 through forwarding addresses sent to us by the postal service. Smaller numbers of address changes were obtained through the other methods listed in table 1: credit bureau searches, directory assistance/Internet queries, telephone calls to participants, calls from participants to the study, and telephone contact with a participants' friends or relatives.
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We compared characteristics of participants whose address changes were identified by the three major methodsquestionnaire, postcard from newsletter, and NCOA. The three groups were similar with regard to age, marital status, geographic region of residence, parity, and cigarette smoking. Women whose address changes were obtained through the NCOA were slightly less educated: 40.3 percent had at least a college degree compared with 45.2 percent who reported the address change themselves either by questionnaire or by postcard.
Overall, 56.5 percent of the women moved at least once, and 1.5 percent moved at least four times. Changes of address were strongly inversely associated with age (figure 1): 71.7 percent of women aged 2129 years at baseline changed addresses at least once and 3.0 percent moved at least four times compared with 43.2 and 0.5 percent of women aged 5069 years. Moving was also strongly inversely associated with completion of the 1997 questionnaire (figure 2). A total of 86.3 percent of the women who had no address changes completed the 1997 questionnaire compared with 68.2 percent of the women with four or more address changes. This comparison was based on all women, including lost participants (address unknown to us after six mailings). Possible reasons for the association of moving with a lower response rate are that women who moved were more likely to become lost or were too busy to fill out health questionnaires. The data support the former explanation: When lost participants were excluded, the inverse association between moving and the completion of the 1997 questionnaire was much weaker, with a response rate of 82.0 percent among women who moved at least four times.
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The use of reminder letters or postcards resulted in very small increases (approximately 3 percent) in the return to the mailing in question. Newsletters were mailed every 6 months. After each mailing, there was a small increase in returns of completed questionnairesless than 2 percent.
Table 2 shows selected characteristics of the women, as reported on the baseline questionnaire according to the number of mailings needed to elicit the return of a completed questionnaire. There was little variation among respondents to the various mailings according to categories of age; providing a Social Security number or the name of a contact person; occupation; whether the woman had a second job; marital status; the number of people living in the household; parity; body mass index; cigarette smoking; recency of the last visit to a physician; region of residence; and history of cardiovascular disease, cancer, or hypertension. The most notable difference was a tendency for women with 17 or more years of education to respond to an earlier mailing: 23.8 percent of respondents to the first mailing had 17 or more years of education compared with 15.8 percent of those who responded after six mailings.
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DISCUSSION |
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In the BWHS, there was a high rate of changing addresses by participants during the 3-year period between enrollment and completion of the first follow-up: 56.5 percent of the participants moved at least once and 1.5 percent moved at least four times. Moving was strongly related to age: The proportion who had moved at least once ranged from a high of 71.7 percent among women aged 2129 years to a low of 43.2 percent among those aged 5069.
To increase response rates, multiple mailings of the follow-up questionnaires were made to women who had not yet responded. A greater proportion of women who had graduated from college than of women with fewer years of education responded to the earlier mailings, but the differences overall in response to particular mailings by educational level were small. Differences in response by other variables, which included age, parity, working at a second job, health status, smoking status, and the number of people living in the household, were even smaller. One might have predicted that women who had second jobs or many children would have tended to respond later, but this was not the case. Similarly, women with health problems were not more or less likely to respond to an early mailing.
Frequent moving was a strong correlate of whether a BWHS participant completed a follow-up questionnaire. The response rate among those who had not moved at all was 86.3 percent, whereas the rate among those who had moved at least four times was 68.2 percent. When we reassessed response rates according to the number of address changes after exclusion of lost participants (women for whom we did not know the correct address after the sixth mailing), the response rate among those who had moved four or more times was 82.0 percent. From this, we conclude that most of the low response among women who had moved multiple times was largely due to failure to reach them rather than to other reasons, such as that women who moved often were too busy to complete questionnaires.
Various methods were used to increase the response rate. These included speaking to participants by telephone, leaving messages on telephone answering machines, sending letters, and sending newsletters. The most effective method was reaching participants at their homes by telephone and conducting interviews. However, this method was very expensive because it was labor intensive and often took numerous calls to reach a woman. The other methods resulted in only small increases in response rates. In our experience, the most cost-effective way to evoke a response was to send multiple waves of questionnaires. Certified mail has been used in numerous studies. We chose not to use this method early in the study because of the burden it imposes on the participant to go to the post office to retrieve the questionnaire if it is delivered when she is not home.
Response rates did not increase materially in the periods immediately after mailings of the biannual newsletters. However, we believe that the newsletters will have the long-term effect of increasing retention rates. Participants have indicated that they were valuable in building rapport between the participants and the investigators and in increasing the faith of the participants in the value of the study. Numerous women have sent questions on study methods or on health issues in their completed questionnaires or have called our toll-free telephone number with questions. While we attempt to answer these questions through individual letters, we also address them in the newsletters. We will continue to use the newsletters to inform the participants about the study methods in addition to bringing them news of study findings and updating them on health issues.
High participation rates are the best safeguard against bias due to selective losses. Some assurance about the possibility of bias can be ascertained from comparisons of the characteristics of respondents with those of nonrespondents and lost participants. BWHS respondents were more likely to have provided their Social Security numbers or contact information than were nonrespondents, possibly indicating that those who responded took the study more seriously. They also had higher levels of education. However, in all other demographic, reproductive, and health characteristics, respondents and nonrespondents were quite similar. The most notable characteristic of lost participants was that they were younger; their higher rate of changing names or addresses resulted in our losing contact with them more frequently.
In conclusion, our early experience in the BWHS indicates that follow-up of a mobile population can be successful. Methods to be used in the future will include certified mailings and the use of e-mail addresses. It should be noted, however, that success comes at a high price because the methods needed to track participants are highly labor intensive and need to be implemented on an ongoing basis.
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ACKNOWLEDGMENTS |
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NOTES |
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REFERENCES |
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