Impact of a Financial Incentive on Case and Control Participation in a Telephone Interview

Patricia F. Coogan  and Lynn Rosenberg

From the Slone Epidemiology Center at Boston University, Boston, MA.

Received for publication November 19, 2003; accepted for publication February 12, 2004.


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The authors investigated the effect of a $5.00 incentive on participation in a telephone interview among cases and controls in an ongoing study of colorectal cancer. Cases and matched community controls were sent a letter introducing the study. One week later, a nurse called to invite the person to participate in a 30-minute telephone interview. After 1 year of data collection (which began in June 2001), the authors began enclosing a $5.00 bill in the initial letter as an incentive. Incentives were mailed to all potential controls. The authors randomized 50% of a subset of cases to receive the incentive. In the year prior to institution of the incentive, 44.2% of 851 controls participated in the interview, as compared with 56.2% of 1,043 controls in the year after the incentive was instituted (p < 0.001). Among cases randomized to receive the incentive (n = 199), 63.8% participated as compared with 68.4% in the nonincentive group (n = 193) (p > 0.05). Among cases aged 60–69 years, the response rate in the incentive group was reduced by 17% (p = 0.03). Thus, among controls, a small monetary incentive appears to promote a feeling of goodwill toward the research. It does not seem to have an equivalent effect among cases, and in the worst case it may insult or annoy some cases who may otherwise have participated.

case-control studies; colorectal neoplasms; epidemiologic methods


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
A number of studies have shown that the use of a monetary incentive in mailed surveys can increase the response rate (1). One meta-analysis of surveys on a variety of topics calculated that use of a monetary incentive increased the response rate twofold (1). In epidemiologic research, a small monetary incentive ($1–$5) increased rates of response to mailed questionnaires in cohort studies (25) and among controls in case-control studies (6, 7). Prior studies have not addressed the question of whether an incentive increases participation in a telephone interview, nor have they addressed the impact of an incentive on case participation. We are conducting an ongoing case-control study designed to evaluate the influence of various medications on colorectal cancer risk. In this paper, we present data for the impact of a $5.00 incentive on participation in a telephone interview among cases and controls.


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The Case-Control Study of Large Bowel Cancer is being conducted among Massachusetts residents aged 50–74 years. Cases with a first diagnosis of adenocarcinoma of the colon or rectum are ascertained through the tumor registrars of approximately 35 participating hospitals and from the Massachusetts Cancer Registry. The study was approved by the institutional review boards of all participating institutions.

Controls are chosen from town books that all cities and towns in Massachusetts must maintain by law. Town books contain the age, name, and address of all town residents aged 18 years or older and are updated approximately every 2 years. A control is matched to each case according to age, sex, and precinct of residence. To be eligible, cases and controls must speak English and must be capable of completing the telephone interview (e.g., not deaf).

The first contact with potential study subjects is an introductory letter, wherein the study is briefly described and the case or control is invited to participate. Cases are not sent letters until permission has been received from their physician to do so. The letter to the cases explains that a human subjects committee approved the release of their name to the study researchers, and the letter to the controls explains that their name was randomly selected from town books. One week after the letter is sent, a nurse-interviewer calls the potential subject to answer any questions and ascertain whether the person wishes to participate in a 30-minute telephone interview. After the initial letter, all follow-up is conducted by telephone. Up to 20 phone calls are made at different times of the day and on different days of the week in an effort to contact the person. If the interviewer encounters an answering machine, she leaves a message with her phone number; numerous messages may be left over a period of several weeks. Interviewers are aware of the case and incentive status of all study subjects so that they can appropriately answer subjects’ questions about the source of subject names and other recruitment issues.

Approximately 1 year after data collection commenced in June 2001, we began mailing a $5.00 bill as an incentive in the initial letter to all controls and a subset of cases in an effort to increase participation rates, particularly among controls. A final sentence was added to the initial letter stating, "$5.00 is enclosed as a gesture of thanks for your help." No other mention was made of the incentive.

We mailed incentives to all potential controls, since the response rate was unacceptably low and prior studies suggested that use of an incentive could be effective. In contrast, the evidence was more limited for the effect of an incentive on cases. Furthermore, the response rate in the first year was appreciably higher among cases than among controls, and use of an incentive required institutional review board approval from all participating hospitals. Thus, for cases, we tested the use of the incentive among cases ascertained from five of the largest hospitals in the study (Cape Cod Hospital, Falmouth Hospital, Massachusetts General Hospital, Brigham and Women’s Hospital, and Boston Medical Center) and from the Massachusetts Cancer Registry. Among those cases, 50 percent were randomized to receive the incentive.

When the interviewers attempt to contact a potential subject by phone, there are four possible outcomes: the interview is conducted (or scheduled); the interview is refused; the subject is ineligible; or the subject cannot be contacted (e.g., incorrect phone number, phone is never answered). We compared the proportions of cases who fell into each of these categories between those randomized to receive the incentive and those randomized to not receive the incentive. For controls, we made comparisons in these categories between controls to whom the letter was sent without $5.00 in the year prior to instituting the incentive (the preincentive group) and those sent the letter with $5.00 in the year after the incentive was instituted (the incentive group). The difference in proportions was assessed with the chi-squared test, and two-sided p values are given.

The data on which these analyses are based were gathered from September 2001 through September 2003.


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The distributions of age and sex were similar among the 851 controls in the preincentive group and the 1,043 controls in the incentive group (table 1). The proportion of controls interviewed was 12 percent higher in the incentive group (56.2 percent) than in the preincentive group (44.2 percent) (p < 0.001), and the proportions of refusals and failure to contact were lower in the incentive group (table 2). Similar proportions of controls were ineligible. As table 2 shows, the incentive increased the proportion of interviewed controls in all age groups; the difference was smallest (5 percent) among controls aged 50–59 years and highest among controls aged 60–69 years (15 percent). The incentive had similar effects in men and women. An analysis confined to the 6 months before and after the incentive was instituted yielded similar results (data not shown).


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TABLE 1. Age and sex distribution of cases and controls who did and did not receive a $5.00 incentive to participate in a telephone interview, Case-Control Study of Large Bowel Cancer, Massachusetts
 

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TABLE 2. Response to a $5.00 incentive to participate in a telephone interview among controls in the Case-Control Study of Large Bowel Cancer, Massachusetts
 
Among the cases, 199 were randomized to receive $5.00 and 193 were randomized to not receive the incentive. The two groups did not differ in terms of age or sex (table 1). A smaller proportion of cases in the incentive group was interviewed (63.8 percent) than in the group that did not receive the incentive (68.4 percent) (table 3). The incentive group was more likely to refuse, more likely to be ineligible, and more likely to have been contacted, although the difference in proportions was statistically significant only for the latter category. Among the youngest cases, participation in the interview was higher in the incentive group (80.3 percent) than in the nonincentive group (69.4 percent), though not significantly so. In the older age categories, the opposite was true, and among cases in their sixties, the 16.7 percent difference in the proportion of people interviewed in the incentive (57.1 percent) and nonincentive (73.8 percent) groups was statistically significant (p = 0.03). The results were similar in men and women; that is, participation was greater in the nonincentive group, but the difference was not statistically significant.


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TABLE 3. Response to a $5.00 incentive to participate in a telephone interview among cases in the Case-Control Study of Large Bowel Cancer, Massachusetts
 
We compared the proportion interviewed with the proportion that refused between cases contacted before the incentive was introduced and those who received the incentive, similar to our comparison of pre- and postincentive controls. The conclusions were the same as those from the randomized analysis (data not shown).

Among both cases and controls, the median number of phone calls made to reach a subject (three) was the same among those who received an incentive and those who did not.


    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The use of a $5.00 incentive increased the proportion of population controls who participated in the telephone interview by 12 percent. This is similar to the findings of two case-control studies of cancer wherein $5.00 increased control response to a mailed questionnaire by 15.3 percent (6) and 20 percent (7). Thus, the incentive appears to have the desired effect among controls of promoting a feeling of goodwill toward the research. Among cases, however, the incentive had no effect or possibly an adverse effect. In particular, among persons aged 60–69 years, the response rate in the incentive group was reduced by 17 percent. Thus, a small monetary incentive does not have the desired effect among cases, and in the worst case it may insult or annoy some cases who may otherwise have participated. Among both cases and controls, the proportion who could not be contacted was lower among persons who received the incentive than among persons who did not. This category includes "passive refusals"—people who screen their calls, refusing to answer when the interviewer calls, and people who ignore repeated messages. The incentive may have motivated subjects to pick up the phone or return the message, whether or not they ultimately participated.

The incentive was assigned randomly among cases, and the incentive and nonincentive groups were similar in terms of age, sex, and ineligibility. For controls, however, the comparison was between the pre- and postincentive groups. Among cases, the results from the randomized analysis and an analysis similar to that conducted among controls (comparing preincentive cases with those who received the incentive) were similar.

Although the control groups were similar with regard to age, sex, and ineligibility, the apparent effect of the incentive would have been inflated if response rates were increasing over the 2 years included in the analysis, unrelated to the incentive. We do not think this is the case, however, because an analysis confined to the 6 months before and after the incentive was instituted yielded results similar to those of the analysis of the years before and after the incentive.

In conclusion, our data indicate that a $5.00 incentive increased control participation in a telephone interview. Many cases are motivated to participate in epidemiologic research so that their experience with cancer may contribute to medical knowledge (8). Our data suggest that a monetary incentive does not enhance this intrinsic motivation or may be detrimental.


    ACKNOWLEDGMENTS
 
This work was supported by National Cancer Institute grant R01 CA87709.


    NOTES
 
Correspondence to Dr. Patricia F. Coogan, Slone Epidemiology Center at Boston University, 1010 Commonwealth Avenue, Boston, MA 02215 (e-mail: pcoogan{at}slone.bu.edu). Back


    REFERENCES
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. Edwards P, Roberts I, Clarke M, et al. Increasing response rates to postal questionnaires: systematic review. BMJ 2002;324:1183.[Abstract/Free Full Text]
  2. John EM, Savitz DA. Effect of a monetary incentive on response to a mail survey. Ann Epidemiol 1994;4:231–5.[Medline]
  3. Little RE, Davis AK. Effectiveness of various methods of contact and reimbursement on response rates of pregnant women to a mail questionnaire. Am J Epidemiol 1984;120:161–3.[ISI][Medline]
  4. Doody MM, Sigurdson AS, Kampa D, et al. Randomized trial of financial incentives and delivery methods for improving response to a mailed questionnaire. Am J Epidemiol 2003;157:643–51.[Abstract/Free Full Text]
  5. Perneger TV, Etter J-F, Rougemont A. Randomized trial of use of a monetary incentive and a reminder card to increase the response rate to a mailed health survey. Am J Epidemiol 1993;138:714–22.[Abstract]
  6. Parkes R, Kreiger N, James B, et al. Effects on subject response of information brochures and small cash incentives in a mail-based case-control study. Ann Epidemiol 2000;10:117–24.[CrossRef][ISI][Medline]
  7. Gilbart E, Kreiger N. Improvement in cumulative response rates following implementation of a financial incentive. Am J Epidemiol 1998;148:97–9.[Abstract]
  8. Savitz DA, Hamman RF, Grace C, et al. Respondents’ attitudes regarding participation in an epidemiologic study. Am J Epidemiol 1986;123:362–6.[ISI][Medline]




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