title: Meta-analysis of randomised trials of monetary incentives and response to mailed questionnaires.
abstract: OBJECTIVE
  To quantify the increase in mailed questionnaire response attributable to a monetary incentive.
  
  
  METHODS
  A 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.
  
  
  METHODS
  Populations in several developed countries, predominantly the USA.
  
  
  METHODS
  85,671 randomised participants from 88 trials.
  
  
  RESULTS
  The 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.
  
  
  CONCLUSIONS
  This 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.
authors.affiliation: Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, London WC1E 7HT, UK. phil.edwards@lshtm.ac.uk
authors.forename: Phil
authors.initials: P
authors.lastname: Edwards
authors.forename: Rachel
authors.initials: R
authors.lastname: Cooper
authors.forename: Ian
authors.initials: I
authors.lastname: Roberts
authors.forename: Chris
authors.initials: C
authors.lastname: Frost
mesh_terms: D006302:Health Services Research / Q000379:methods*; D006801:Humans; D016015:Logistic Models; D009042:Motivation; D010358:Patient Participation / Q000191:economics; D016032:Randomized Controlled Trials as Topic; D012201:Reward; D011795:Surveys and Questionnaires / Q000191:economics