Goldenberg et al., 2009). Singer concluded that incentives appear to increase response among those who have previously refused, but not among those who have previously cooperated (Zagorsky and Rhoton, 2008).

In a study by Jaeckle and Lynn (2008) of incentive payments in a U.K. longitudinal study, the researchers found that (1) attrition was significantly reduced by incentives in all waves, (2) the attrition was reduced proportionately among subgroups and so did not reduce attrition bias, (3) the effect of the incentive decreased across waves, (4) incentives increased item nonresponse, but (5) there was a net gain in information.

The NLSY97 has been a rich source of analysis of the effects of incentive payments on participation in a longitudinal survey because, from the beginning, NLSY management has had discretion over the level of incentives to be offered to participants. The amount of the incentive has also been adjusted on an experimental basis. In an early study conducted in NLSY97 Round 4 and extended into Round 5, Datta et al. (2001) found that sample members who were paid $20 had higher participation rates than those paid $10 or $15. However, there were no measurable effects on data quality from the higher level of incentives.

Subsequent NLSY97 experiments found that higher incentives had a particular effect on bringing those who dropped from prior rounds back into participation in later rounds. Pierret et al. (2007) studied the results of the incentive experiments and concluded that incentives moderately increased response rates and had a greater impact on those respondents who did not participate in the previous round relative to those who did participate.

An incentive experiment was conducted as part of the 2000 wave of HRS, in which the incentive amount was increased from $20 to $30 or $50. Rodgers (2011) found an improvement in response rates as the incentive increased. A lowered incentive amount of $40 in subsequent rounds or waves did not result in lowered response rates. He also found a statistically significant decrease in item nonresponse among respondents receiving larger incentives.

Other Findings on Incentive Effects

Two experiments failed to find a role for interviewers in mediating incentive effects. Singer et al. (2000) kept interviewers blind to households’ receipt of incentives in one condition but not in another and found that there were no differences in incentive effects between the two conditions. Lynn (2001) randomly offered promised incentives to half of each interviewer’s assigned households and then asked interviewers how useful they thought the incentives had been. Interviewers’ judgments were almost uniformly negative, but incentives had significant positive effects

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