Partecitation in CATI surveys: traditional nonresponse adjustmentsversus propensity score matching in reducing nonresponse bias

In CATI surveys, several stages of the survey process can be identified. Participation at each stage can be analysed as dependent on the outcome of the previous stage. For a specific type of CATI surveys - e.g., a survey on general population with sample selected from the population register and telephone numbers of the selected units obtained from a merge with a telephone directory - the following stages can be identified: pre-recruitment (merge with the telephone directory), contact (call scheduling and call backs), interview. The initial sample is often modified as a result of the pre-recruitment stage. The identification of the telephone number of the selected individual is not always successful: when the number is not identified the individual is substituted with another with the same characteristics and a known telephone number. This updated sample is contacted by telephone following the rules of the call scheduling. The units not contacted after a fixed number of tentative telephone calls (missed contacts) and the units who refuse the interview (refusals) are often substituted in order to maintain the planned sample size. The underlying assumption is that survey substitutes do not differ systematically from the individual they replace. However, since substitutes are, by definition, telephone subscribers contacted by phone and respondents they may differ from the nonrespondents and may bias survey estimates. This paper uses propensity score matching methods to find out possible substitutes without requiring that substitutes be perfect replacement for non respondents and to compensate for the self-selection bias occurred in the above stages of a CATI survey on individuals. In the paper, the pre-recruitment, contact and interview stages are distinguished in a CATI survey targeting the general population aged 20-75 of the Municipality of Florence. Using a parallel population register of the Municipality of Florence, the probability of being in each stage of the CATI survey is estimated, based on a vector of covariates measured both in the register and in the survey. The aim of the research is to illustrate for each of the previous stage the performance of the propensity score matching adjustment relative to the commonly used individual substitutions method in order to evaluate survey results and their accuracy, develop adequate weighting procedures and strategies for reducing non response error.