Effects of small incentives on survey response fractions: randomised comparisons in national alcohol surveys conducted in New Zealand

We experimentally evaluate inexpensive interventions to increase response fractions in two alcohol surveys. Residents on the New Zealand General and Maori electoral rolls were randomized to receive a survey pack with or without an offer of entry to a $500 prize draw. Subsequent randomization of sample members who did not initially respond allowed estimation of effects of offering a $5 donation to charity as an incentive to respond. Offering prize draw entry did not significantly increase responses in either population. Contrary to expectation, promising a $5 donation to non-respondents reduced subsequent responding in the group previously offered the prize draw incentive.