INCREASING RESPONSE RATES IN TELEPHONE SURVEYS - A RANDOMIZED TRIAL

Background. Sampling frames and mode of contact and administration of questionnaires are important factors contributing to response rates an d selection bias in population-based research. The purpose of this pap er is to evaluate whether contact by mail before contact by telephone increases response rate, and to assess the concurrent validity of tele phone surveys for collecting health research and service data. Methods . Two thousand households were randomly selected from electronic white pages. Half were randomly allocated to receive or not to receive an e xplanatory letter before telephone contact. Interviewers were blinded to whether a household received a letter. Respondents aged 18 years or over were randomly selected from within each household using a Kish g rid and interviewed by telephone.Results. The overall response rate wa s 68 per cent [confidence interval (CI) 66-70]. The response rate of t hose who received the letter was 76 per cent (CI 73-79), and of those who did not receive the letter was 60 per cent (CI 56-63). Use of the Kish grid to select randomly a respondent decreased the response rate by less than 10 per cent. The internal validity of the data was as fol lows: in a 10 per cent sub-sample, the Kish grid had been correctly ap plied in 93 per cent of households, and in 99 per cent of households t he exclusion criteria had been correctly adhered to. The external vali dity was as follows: comparisons with data obtained from the same refe rence population using similar instruments administered face-to-face r evealed no meaningful or significant differences in population estimat es. Conclusions. Mail-out before telephone contact greatly increases r esponse rates at low cost. Telephone surveys can yield valid, useful d ata for health research and service evaluation.