In an effort to counter increasing tendencies toward noncooperation, survey organizations are offering incentives to respondents with increasing frequency, some at the outset of the survey, as has been done traditionally in mail surveys, and some only after the person has refused, in an attempt to convert the refusal.
The use of incentives has a long history in mail surveys (for reviews, see Armstrong, 1975; Church, 1999; Cox, 1976; Fox et al.,1988; Heberlein and Baumgartner, 1978; Kanuk and Berenson, 1975; Levine and Gordon, 1958; Linsky, 1975; Yu and Cooper, 1983). In such surveys, incentives are one of two factors, the other being number of contacts, that have been found to increase response rates consistently.
A meta-analysis of the experimental literature on the effects of incentives in mail surveys by Church (1999) classifies incentives along two dimensions: whether the incentive is a monetary or nonmonetary reward, and whether it is offered with the initial mailing or made contingent on the return of the questionnaire. Analyzing 38 mail surveys, Church concluded that:
- Prepaid incentives yield higher response rates than promised incentives;
- The offer of contingent (promised) money and gifts does not significantly increase response rates;
- Prepaid monetary incentives yield higher response rates than gifts offered with the initial mailing; and
- Response rates increase with increasing amounts of money.
Studies using prepaid monetary incentives yielded an average increase in response rates of 19.1 percentage points, representing a 65-percent average increase in response. Gifts, on the other hand, yielded an average increase of only 7.9 percentage points. The average value of the monetary incentive in the mail surveys analyzed by Church was $1.38 (in 1989 dollars); the average value of the gift could not be computed, given the great diversity of gifts offered and the absence of information on their cost. Reports similar to those of Church are reported by Hopkins and Gullikson (1992).
Incentives are also used increasingly in telephone and face-to-face surveys, and the question arises as to whether their effects differ from those found consistently in mail surveys. A meta-analysis of 39 experiments by Singer et al. (1999a) indicates that they do not, although the percentage point gains per dollar expended are much smaller, on average (and the levels of incentives paid significantly higher), than those reported by Church. Their main findings are as follows:
- Incentives improve response rates in telephone and face-to-face surveys, and their effect does not differ by mode of interviewing. Each dollar of incentive paid results in about a third of a percentage point difference between the incentive and the zero-incentive condition. As in the analyses by Church (1999) and Yu and Cooper (1983), the effects of incentives are linear: within the range of incentives used, the greater the incentive, the greater the difference in response rates between the lowest and the higher incentive conditions.
- Prepaid incentives result in higher response rates than promised incentives, but the difference is not statistically significant. However, prepaid monetary incentives resulted in significantly higher response rates in the four studies in which it was possible to compare prepaid and promised incentives within the same study.
- Money is more effective than a gift, even controlling for the value of the incentive.
- Increasing the burden of the interview increases the difference in response rates between an incentive and a zero-incentive condition. However, incentives have a significant effect even in low-burden studies.
- Incentives have significantly greater effects in surveys where the response rate without an incentive is low. That is, they are especially useful in compensating for the absence of other motives to participate. They are also most effective in the absence of other persuasion efforts. A number of studies have found that the difference in response rates between the group that received the incentive and the group that did not receive an incentive diminished after repeated follow-up attempts.
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