Learning, understanding, and motivation effects on “don’t know” in panel surveys

Whether a changed response quality in panel surveys is due to general learning, a higher motivation, or panel conditioning is difficult to analyze. At least, there is some agreement that response quality changes over time depends largely on the person groups considered. In this paper we analyze whether different person-groups change “don’t know” answers to the same political questions in different ways due to different time in the panel, survey question understanding, and motivation effects. Using data from a nationally representative telephone panel, we find that “don’t know” answers generally decreases with time in the panel, especially for respondents new to Switzerland, young people, and foreigners answering in a foreign language. These effects are in parts due to a  better question understanding. More motivation is effective for respondents new to Switzerland, those with a lower education, young, and old people.  Older people, however, are the only person group with increased “don’t know” answers and with a significantly decreasing question understanding over time.