The Danish National Transport Survey has been running since 1993 with about 14.000 telephone interviews per year, but the observed average distance travelled (ADT) is not developing as it should be expected from other data sources, for instance car traffic counts. A more thorough investigation showed that the most important change was an increase in the share staying at home on the interview day from around 15% of the interviewed in 1997 to around 25% in 2001. The trip rate for persons with trips decreased in the same period. None of thiese changes is expected. The ADT on the other hand did not decrease for persons with trips. From 2002 the trip rate and zero trip rate changed again to the 1998 level. The purpose of the paper is to find explanations to the changes over the years by the performance of the interviewers, and by changes in the data collection methodology. Problems with soft refusal are discussed and changes related to introduction of cell phones as interview media is analysed. The main reason for the observed changes in the zero trip rate and the trip rate seems to be differences in interviewer performance. The results emphasise the importance of intensive monitoring and quality control of interviewer results. Interviewer fatigue seems to be important to take into account when carrying out such surveys over a long period of time. Changes in data collection methodology contribute less to explaining the observed changes, since there has been a relative stable distribution of interviews over the day and week. However, the results indicate the importance of maintaining a stable distribution of interviews over the day and week, since interview results are highly dependent on both. Changes in this distribution may invalidate comparisons over time. Introduction of mobile telephones in the calling has changed, both contact pattern and the result of the interviews. For the covering abstract please see ITRD E135207.
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