For a variety of reasons, including those discussed in the previous section, prepaid incentives to everyone in the sample may be preferable to refusal conversion or other differential payments.
One reason is that interviewers like them. Knowing the household is in receipt of an advance payment, modest though it may be, interviewers feel entitled to ask the respondent to reciprocate with an interview. Furthermore, prepaid incentives are equitable. They reward equally everyone who happens to fall into the sample, and they reward them for the right behavior that is, for cooperation, rather than refusal. Both of these advantages are likely to make modest prepaid incentives an attractive alternative to refusal conversion payments in many types of surveys. There is also indirect evidence that the use of refusal conversion payments to persuade reluctant respondents leads to increasing reliance on such payments within an organization, in all likelihood because of their effects on interviewer expectations.
Still, the question arises whether such incentives are cost effective. It would appear that paying a small number of refusal conversion payments to reluctant respondents would be cheaper than paying everyone, even if those initial payments are smaller.
Several studies have concluded that prepaid incentives are cost effective in mail surveys. For such surveys, the comparison ordinarily has been among incentives varying in amount or in kind, or in comparison with no incentive at all, rather than with refusal conversion payments. Two recent investigations of cost effectiveness, by James and Bolstein (1992) and by Warriner et al. (1996), have included information on the relative effectiveness of various incentives. James and Bolstein (1992) found that a prepaid incentive of $1 was the most cost effective, yielding nearly as high a return as larger amounts for about one-quarter of the cost. Warriner et al. (1996:9) conclude that for their study, a $5 prepaid incentive was the optimal amount, resulting in a saving of 40 cents per case (because the same response rate could be achieved as in a no-incentive, two-follow-up condition). The $2 incentive resulted in costs per case only a dollar less than the $5 incentive, while yielding a response rate 10 percentage points lower. Similar findings have been reported by Asch et al. (1998) in a mail survey of physicians.
For interviewer-mediated studies, as noted earlier, the comparison is much more likely to be with refusal conversion payments. The answer is likely to depend on the nature of the study and the importance of a high response rate, on how interesting the study is to respondents (i.e., how many of them are willing to participate even without a prepaid incentive), on whether prepaid incentives reduce the effort required, and on a variety of other factors.
Several face-to-face surveys have reported that promised monetary incentives are cost effective. Berlin et al. (1992), for example, reported that use of a $20 promised incentive in a field-test experiment with the National Adult Literacy Survey, which entails completion of a test booklet by the respondent, resulted in cost savings on a per interview basis when all field costs were taken into account. Similarly, Chromy and Horvitz (1978) reported (in a study of the use of monetary incentives among young adults in the National Assessment of Educational Progress) that when the cost of screening for eligible respondents is high, the use of incentives to increase response rates actually may reduce the cost per unit of data collected.
Singer, Van Hoewyk, and Couper(11) investigated this problem in the Survey of Consumer Attitudes (SCA). They found that a $5 incentive included with an advance letter significantly reduced the number of calls required to close out a case (8.75 calls when an incentive was sent, compared with 10.22 when it was not; p=.05), and significantly reduced the number of interim refusals (.282 refusals when an incentive was sent, compared with .459 when it was not). As expected, there was no significant difference between the incentive and the no-incentive condition in calls to first contact. The outcome of the first call indicates that compared with the letter only, the addition of a $5 incentive results in more interviews, more appointments, and fewer contacts in which resistance is encountered.
Given the size of the incentive and the average cost per call aside from the incentive, sending a prepaid incentive to respondents for whom an address could be obtained was cost effective for the SCA. However, as we have tried to indicate, this conclusion depends on the size of the incentive as well as the structure of other costs associated with a study for a given organization, and should not be assumed to be invariant across organizations and incentives.
An argument that can be raised against the use of prepaid incentives is that they may undermine more altruistic motives for participating in surveys. Indeed, we have found that prepaid incentives have smaller effects on survey participation for people who score high on a measure of community activism (Groves et al., 2000) than on people who score low on this characteristic. But this is because groups high in community activism already respond at a high rate. There is no evidence (because we did not test this hypothesis) that people high on community activism who are offered a prepaid incentive respond at a lower rate than they would have had they not been offered the incentive, nor do we know whether such an effect would appear on a later survey. Although anecdotal evidence shows that some people are offended by the offer of an incentive, going so far as to return the incentive to the survey organization, by all accounts such negative reactions are few.
Prepaid incentives have been common in mail surveys for many years, although the amounts used are ordinarily quite modest (see Church, 1999). We suspect that the use of such incentives will increase in interviewer-mediated surveys as well. Such incentives are likely to be especially appropriate when other reasons that might move potential respondents to participate are weak or lacking, and when the names and addresses (or telephone numbers) of such potential respondents are known.
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