they choose to participate. Based on a content analysis of open-ended responses, their study of 140 participants in 5 waves of a German Methods Panel identifies 3 pure types of participants: (1) those who respond for altruistic reasons (e.g., the survey is useful for some purpose important to the respondent, or the respondent is fulfilling a social obligation—31 percent of respondents); (2) those who respond for survey-related reasons (e.g., they are interested in the survey topic, or find the interviewer appealing—38 percent); and (3) those who cite what the authors call personal reasons (e.g., they promised to do it—30 percent). In reality, of course, most people participate for a variety of reasons.

More recently, Groves et al. (2000) outlined a theory describing the decision to participate in a survey as resulting from a series of factors—some survey specific, such as topic and sponsorship, others person specific, such as concerns about privacy, still others specific to the respondent’s social and physical environment—each of which may move a particular person toward or away from cooperation with a specific survey request. Furthermore, these factors assume different weights for different persons, and they become salient for a specific individual—the potential respondent—when an interviewer calls to introduce the survey and request participation.

From this perspective, monetary as well as nonmonetary incentives are an inducement offered by the survey designer to compensate for the relative absence of factors that might otherwise stimulate cooperation—for example, interest in the survey topic or a sense of civic obligation. Although other theoretical frameworks such as social exchange theory (cf. Dillman, 1978), the norm of reciprocity (Gouldner, 1960), and economic exchange (e.g., Biner and Kidd, 1994) also can be used to explain the effectiveness of incentives, the present perspective is able to account for the differential effects of incentives under different conditions (e.g., for respondents with differing interest in the survey topic or with different degrees of community activism) in a way that other theories cannot easily do.

Incentives and Hard-to-Reach Populations

As indicated above, members of a group may be hard to interview both because they are difficult to locate or to find at home and because they have little motivation to participate in a survey. There is no empirical evidence that incentives are helpful in overcoming the first problem in a random digit dial (RDD) survey, nor any theoretical justification for believing that they would or should be. Thus, if the primary problem is one of finding people at home for such a survey, incentives may not be very useful. However, an experiment by Kerachsky and Mallar (1981) with a sample of economically disadvantaged youth suggests that prepayment may be helpful in locating members of a list sample, especially in later waves of a longitudinal survey. One reason, apparently, is that prepayment (and perhaps promised incentives from a trusted source) may be useful in persuading friends or relatives to forward the survey organization’s advance



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