Identifying different types of observed immobility within longitudinal travel surveys

In every travel survey there are respondents who did not report any trips on a given day. It is important to distinguish truly immobile respondents from respondents who simply stated that they were immobile (soft refusers). Three methods are proposed in this paper to identify possible soft refusers and tested on the Netherlands Mobility Panel. Identification is done based on attrition, determinants of out-of-home activity, and questionnaire response behaviour. All three methods identify a group of respondents who are at risk of exhibiting soft refusal. By combining the three methods, approximately 5% to 10% of respondents were found to be at high risk of exhibiting soft refusal, and their data should therefore be treated with extra caution.