Effects in Interviewer-Mediated Surveys
A meta-analysis of 39 experiments by Singer et al. (1999) found results for surveys using interviewers that were similar to those in mail surveys, although the effects of incentives were generally smaller. The analysis of 114 RDD surveys by Holbrook et al. (2008) found that surveys offering incentives had significantly higher response rates than those offering no incentives; the effect came mainly from a reduction in refusals. The 2008 analysis by Cantor et al. of 23 RDD experiments found that:
• a prepayment of $1 to $5 increased response rates from 2 to 12 percentage points;
• larger incentives led to higher response rates;
• the effect of incentives has not declined over time, but baseline response rates have dropped substantially;
• incentives at refusal conversion had about the same effect as those sent at initial contact; and • promised incentives of $5 and $25 did not increase response rates compared to no incentives, but promising larger incentives sometimes did.
These findings are generally consistent with other experiments involving interviewer-mediated surveys, including face-to-face surveys.
Effects in Cell Phone Surveys
Brick et al. (2007) conducted a dual-frame survey including both cell phones and landlines that include an incentive experiment. This study used two promised incentive conditions ($10, $5) and two message conditions (sample members notified of the survey and incentive by text messaging, or not notified). They found that the $10 group had a higher screener response rate than the $5 group as well as a higher cooperation rate. The message had no effect on either screener or survey response rates, and there were no interaction effects.
Incentives in Longitudinal Studies
Longitudinal surveys have special issues, because incentives are usually part of a larger motivational package designed to retain respondents. As in cross-sectional studies, initial survey round incentives have been found to increase response rates, usually by reducing refusals but sometimes by reducing non-contacts (e.g., McGrath, 2006). Some studies suggest that an initial payment may continue to motivate participation in subsequent waves (Singer and Kulka, 2001; McGrath, 2006; Creighton et al., 2007;