The Effects of Material Incentives in Mail Surveys: Two Studies

Two studies were undertaken on the effects of including a material nonmonetary incentive in mail surveys, using various samples of the general public in the Netherlands. The results show that nonmonetary incentives produce a higher initial response rate, but follow-ups reduced the effect of the incentive to a nonsignificant ratio. Inclusion of the incentive produced no response bias and little volunteer bias. Results from these studies offer a possible explanation for why past studies on nonmonetary incentives have often shown positive effects: they were conducted using methods that produced low response rates. When methods that produce high response rates are used, the effect of nonmonetary incentives on response rate disappears. Finally, the use of monetary incentives in mail surveys with a high base response is discussed. Anton J. Nederhof is a Research Fellow at the Center for Social Science Research, University of Leyden, The Netherlands. This paper was written while the author was a Visiting Fulbright Scholar at the Departments of Rural Sociology and Sociology, Washington State University, Pullman, Washington. The author wishes to thank Don A. Dillman and Leo Th. J. van der Kamp for their comments on an earlier draft of this paper. Public Opinion Quarterly Vol. 47:103-111 ? ) 1983 by the Trustees of Columbia University Published by Elsevier Science Publishing Co., Inc. 0033-362X/83/0047-103/$2.50 This content downloaded from 207.46. 13.127 on Fri, 14 Oct 2016 04:09:15 UTC All use subject to http://about.jstor. org/terms 104 ANTON J. NEDERHOF A number of studies have shown that nonmonetary incentives raised returns relative to a control group (Brennan, 1958; Goodstadt et al., 1977; Hansen, 1980; Houston and Jefferson, 1975; Watson, 1965; Whitmore, 1976), although not all increases were statistically significant. All experiments were geographically limited to the United States. Most of the studies sampled specific homogeneous groups, making generalization of results difficult or unwarranted (Cook and Campbell, 1979). In addition, response rates of no-incentive control groups were generally low. No experiments have been done with control groups with high response rates, such as those obtained by implementing a set of methods developed by Dillman (1978), resulting in returns of more than 70 percent. Thus, it remains unclear whether nonmonetary incentives would lead to a higher response rate in cases when a relatively high base rate can be obtained without use of the incentive. Another issue which deserves attention is the possibility that the use of incentives affects the validity of findings. First, incentives may induce some (groups of) respondents to participate, and others not. This type of bias will be called volunteer bias (Rosenthal and Rosnow, 1975). A second type of bias may exist independent of the first type: incentives may affect subjects' answers. This type of bias is called response bias. Both types of bias have been found with monetary incentives (Gelb, 1975; Rush et al., 1978). Little is known about the biasing effects, if any, of nonmonetary incentives (Whitmore, 1976; Brown and Coney, 1977). The two present studies, executed in a European country, addressed three issues: (1) the effects of a nonmonetary incentive on response rate under conditions of a high base response rate, (2) inducement of response and volunteer bias by inclusion of an incentive, and (3) the cross-cultural effectiveness of methods developed in the United States.