Impact of Nonresponse and Weighting in a Swedish Survey

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Susanne Gustafsson

Results from travel surveys, together with traffic monitoring programs, serve as a basis for policy decisions and evaluations. Correct interpretation of the survey results is therefore of vital importance, and the impact of different sources of error needs to be investigated. This paper presents a study of nonresponse errors in a Swedish travel survey. Survey methodologists have addressed the problem of nonresponse for a long time, and several methods have been developed to reduce possible bias. Particularly common are different weighting methods based on auxiliary information. However, the performance of these techniques is based on how well the chosen auxiliary variables can explain the response behavior. The aims of the present study were to (a) investigate whether the mobility of nonrespondents differs from the mobility of respondents and (b) determine whether weighting reduces possible nonresponse bias. A weighting technique called calibration was used. The travel survey was based on a mail questionnaire, and the response rate was about 53%. Nonrespondents were contacted by telephone and asked a selected number of questions from the original questionnaire. Results show that mobility did differ between respondents and nonrespondents for some of the travel modes. The effect of calibration was generally small and inconsistent; the bias was reduced in some cases and increased in others. Nevertheless, calibration is recommended if procedures for calculating the weights are readily available.

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