• Phil Edwards, Rachel Cooper, Ian Roberts, and Chris Frost.
• Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, London WC1E 7HT, UK. phil.edwards@lshtm.ac.uk
• J Epidemiol Community Health. 2005 Nov 1; 59 (11): 987-99.

Study ObjectiveTo quantify the increase in mailed questionnaire response attributable to a monetary incentive.DesignA systematic search for randomised controlled trials of monetary incentives and mailed questionnaire response was conducted. For each trial identified, logistic regression was used to estimate the odds ratio for response per 0.01 dollars incentive increase. Odds ratios were pooled in a series of random effect meta-analyses stratified by the minimum and maximum amounts offered. Piecewise logistic regression was used to estimate the odds ratio for response per $0.01 increase given in each of five incentive ranges.SettingPopulations in several developed countries, predominantly the USA.Participants85,671 randomised participants from 88 trials.Main ResultsThe pooled odds ratios for response per 0.01 dollars incentive decreased monotonically as the maximum amount of incentive offered increased. The piecewise logistic regression model estimated that for incentive amounts up to 0.50 dollars, each additional 0.01 dollars increased the odds of response by about 1% (pooled OR = 1.012, 95%CI 1.007 to 1.016). The effects on response above 0.50 dollars were smaller and decreased monotonically in the ranges: 0.50-0.99 dollars, 1-1.99 dollars, 2-4.99 dollars, 5.00 dollars and over, but remained statistically significant up to 5 dollars.ConclusionsThis meta-analysis of the best available evidence shows that monetary incentives increase mailed questionnaire response. Researchers should include small amounts of money with mailed questionnaires rather than give no incentive at all.

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