How to increase response rates to postal questionnaires.
METHODS
Fourteen electronic databases were searched, namely Cochrane Controlled Trials Register, CINAHL (Cumulative Index to Nursing & Allied Health Literature), ERIC (Education Resources Information Center) PsycLit, Dissertation Abstracts, Medline, Embase, Social Science Citation Index, Science Citation Index, Social Psychological Educational Criminological Trials Register, EconLit, Sociological Abstracts, Index to Scientific and Technical Proceedings, and National Research Register. Literature reviews and meta-analyses were inspected for eligible trials and two journals (Public Opinion Quarterly and American Journal of Epidemiology) were searched by hand.


METHODS
Studies included were randomised controlled trials of methods to increase response rates to postal questionnaires.


METHODS
Data describing trial participants, intervention, number of individuals randomised to intervention and comparison groups, and allocation concealment was extracted. For each strategy, pooled odds ratios and 95% confidence intervals in a random-effects model were estimated. Evidence for selection bias was assessed using Egger's weighted regression method, Begg's rank correlation test and funnel plots. Heterogeneity in trial odds ratios was assessed using a chi-square test at a 5% significance level and the degree of inconsistency between trial results was quantified using the I(2) statistic (The I(2) statistic measures the proportion of the variation across studies that is because of heterogeneity. It is calculated by dividing the heterogeneity chi-square minus the number of degrees of freedom by the heterogeneity chi-square, and then multiplying by 100.).


RESULTS
From 372 eligible trials that were included, 98 different ways of increasing response rates to postal questionnaires were evaluated. For 62 of these strategies, the combined trials included over 1000 participants. There was substantial heterogeneity in the trial results from half of the strategies. Strategies found to response are shown in table 1, while three strategies were found to decrease response, questionnaire included questions of a sensitive nature (odds ratio 0.94, 95% CI 0.88-1.00; heterogeneity P =0.51; I(2) 0%), questionnaires began with the most general questions (odds ratio 0.80, 95% CI 0.67-0.96; heterogeneity Not calculated), Participants were offered the opportunity to opt out of the study (odds ratio 0.76, 95% CI 0.65-0.89; heterogeneity P =0.46, I2 0%).


CONCLUSIONS
Health researchers using postal questionnaires can increase response rates using the strategies shown to be effective in this systematic review.
Richards
Derek
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Centre for Evidence-based Dentistry, Oxford, UK.