Follow-up of a Large Cohort of Black Women

Cordelia Russell1, Julie R. Palmer1, Lucile L. Adams-Campbell2 and Lynn Rosenberg1

1 Slone Epidemiology Unit, Boston University School of Medicine, Brookline, MA.
2 Howard University Cancer Center, Howard University College of Medicine, Washington, DC.


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
High retention rates in follow-up studies reduce the potential for biased data due to selective losses. The Black Women's Health Study began in 1995 when 64,500 participants aged 21–69 years enrolled by completing postal health questionnaires. Follow-up is carried out biennially. On the basis of data collected between enrollment and completion of the first follow-up, the authors assessed the usefulness of various follow-up methods and compared the characteristics of respondents, nonrespondents, and women lost to follow-up because of an unknown address. The 1997 questionnaire was completed by 82.8% of the participants. The study population was highly mobile: 56.5% moved at least once, and 1.5% moved at least four times. Moving was associated with younger age: A total of 71.7% of participants aged 21–29 years moved at least once compared with 43.2% of women aged 50–69. The most successful and cost-effective method for eliciting completed questionnaires from participants was sending multiple waves of questionnaires. Telephone calls to nonrespondents were successful but were highly labor intensive. Demographic and health characteristics of the women were similar regardless of which mailing was completed, except that early respondents had higher levels of education. Respondents were more highly educated and older than were nonrespondents and lost subjects but were quite similar in all other characteristics. These data suggest that follow-up of a mobile population of African-American women can be successful.

blacks; epidemiologic methods; follow-up studies; women

Abbreviations: BWHS, Black Women's Health Study; NCOA, National Change of Address.


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In 1976, the Nurses' Health Study established a model for a new type of prospective cohort study, in which geographically dispersed participants are followed at regular intervals through postal questionnaires (1Go). The relatively modest cost of collecting exposure and outcome information by mail made it economically feasible to conduct very large studies. In 1995, we began the first large-scale prospective follow-up study of African-American women, the Black Women's Health Study (BWHS), with follow-up largely through postal questionnaires.

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 (2GoGo–4Go). In a study that included both Black and White Americans, attrition was higher among the Black Americans (5Go). 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 (6Go). 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.


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Establishment of the cohort
The aim of the BWHS is to assess risk factors for breast cancer and other illnesses among US Black women. In 1995, 64,500 Black women aged 21–69 years were enrolled through postal questionnaires that had been mailed to subscribers to Essence magazine, members of selected Black professional organizations, and friends and relatives of respondents. Approximately equal proportions were from the Northeast, South, Midwest, and West, with the largest numbers from California, Georgia, Illinois, Indiana, Louisiana, Maryland, Massachusetts, Michigan, New Jersey, New York, South Carolina, Virginia, and the District of Columbia. The median age of the respondents was 38 years.

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 2–3 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.


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The 1997 follow-up of the BWHS cohort began in March 1997, 2 years after enrollment in 1995. At the end of the follow-up in December 1998, 53,419 participants (82.8 percent) had completed the 1997 questionnaire, 288 (0.4 percent) were known to have died, 3,095 (4.8 percent) were lost (i.e., their addresses were unknown to us), and there were 7,544 nonrespondents (11.7 percent) and 179 refusals (0.3 percent). The deaths were ascertained through the National Death Index (n = 102), the Social Security Death Master File (n = 57), family members and friends (n = 77), and the postal service or credit bureaus (n = 52).

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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TABLE 1. Source for address changes between March 1995 and December 1998, Black Women's Health Study

 
Some of the information sources were inexpensive. For example, our NCOA vendor charged a flat rate of $500 for an entire search of our cohort, and directory assistance searches cost only the price of a telephone call. The credit bureau searches were somewhat more expensive, at $1.50 per name. Most expensive was the strategy of contacting friends and relatives to locate lost participants. Multiple calls were needed in most instances, and only 30.3 percent of the attempts were successful compared, for example, with a 63.9 percent success rate for credit bureau searches.

We compared characteristics of participants whose address changes were identified by the three major methods—questionnaire, 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 21–29 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 50–69 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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FIGURE 1. Changes of address by age of the women, Black Women's Health Study, March 1995 to December 1998.

 


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FIGURE 2. Response rate by changes of address, Black Women's Health Study, March 1995 to December 1998.

 
Of the 53,419 women who completed the 1997 questionnaire, 54.9 percent completed it after one mailing, 18.6 percent after two mailings, 11.0 percent after three, 9.9 percent after four, 3.6 percent after five, and 2.0 percent after six. We tried various methods of enhancing response rates. Telephoning women who had not completed questionnaires after multiple mailings was an effective, but labor-intensive, method of increasing response: About one third (34.4 percent) of the women reached by telephone agreed to an interview (n = 1,180), but numerous telephone calls were required to reach many of them. We left reminder messages on the answering machines of approximately 6,500 women who had not responded after multiple mailings, and about 15 percent (n = 984) returned completed questionnaires. The cost of telephone interviews was appreciably greater than that of questionnaire mailings. We calculated the cost per returned questionnaire in a particular mailing by summing the cost of printing and processing the questionnaires, postage for outgoing questionnaires, and postage for returned, completed questionnaires and dividing by the number of questionnaires returned from that mailing. The cost per returned questionnaire was $3.15 for the first mailing, $5.07 for the second, $5.16 for the third, $5.12 for the fourth, $9.03 for the fifth, and $13.40 for the sixth. The cost for a telephone interview included the staff time required to reach a participant, telephone charges for a 20-minute interview, telephone charges for attempts made to reach the participant, and calls to contacts to determine a new address or telephone number. Telephone interviews cost $66 per successfully obtained interview. We rarely reached participants during daytime hours on weekdays. Therefore, we concentrated on evening and weekend calls.

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 questionnaires—less 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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TABLE 2. Characteristics of respondents according to the number of questionnaire mailings required to elicit a completed questionnaire*, Black Women's Health Study, March 1995 to December 1998

 
Respondents to the 1997 follow-up were compared with nonrespondents and lost participants (table 3). Because the lost participants were appreciably younger than the respondents and nonrespondents, we standardized the percentages shown in table 3 to the overall age distribution of the cohort. The biggest differences between respondents and nonrespondents were that the former had more often completed college or a higher level of education (45.8 vs. 34.7 percent), had professional or technical occupations (55.6 vs. 47.2 percent), and supplied their Social Security number (80.2 vs. 69.6 percent) or contact information (60.7 vs. 46.2 percent). Differences according to the presence of a second job, marital status, number of people in the household, parity, recency of the last visit to a physician, smoking status, region of residence, and health status were small. Compared with respondents and nonrespondents, lost participants were younger; were more frequently separated, widowed, or divorced; had higher parity; were more frequently current smokers; and more frequently resided in the West. They were similar to respondents in the proportion who provided Social Security numbers and similar to nonrespondents in occupational and educational level.


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TABLE 3. Characteristics of respondents, nonrespondents, and lost participants*, Black Women's Health Study, March 1995 to December 1998

 

    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Many of the large-scale, prospective follow-up studies of geographically dispersed populations launched in the last 25 years have been of White health professionals (1Go, 7GoGoGo–10Go). The BWHS is the first large-scale follow-up study of the health of US Black women. Most of the participants are not health professionals, and there is a wide range of educational achievement, with more than half having completed fewer than 16 years of education. It has been observed that participation in health research studies increases with increasing education (11Go, 12Go) and that members of minority groups tend to have lower participation and higher dropout rates than do White persons (11GoGoGo–14Go). Thus, the experience of the BWHS in tracking its participants may have relevance for other studies attempting to follow persons who are members of minority groups, who may not be health professionals, or who may have lower levels of education.

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 21–29 years to a low of 43.2 percent among those aged 50–69.

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.


    ACKNOWLEDGMENTS
 
Supported by grant R01 CA58420 from the National Cancer Institute.


    NOTES
 
Reprint requests to Cordelia Russell, Slone Epidemiology Unit, Boston University School of Medicine, 1371 Beacon Street, Brookline, MA 02446 (e-mail: drussell{at}slone.bu.edu).


    REFERENCES
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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Received for publication March 8, 2001. Accepted for publication May 25, 2001.