Abstract
This article is based on a secondary analysis of non-responses (NR) and “don't know” (DK)-responses in two French surveys on sexual behaviour. The analysis is situated within a problem framework focusing on dissociating the respondent's ignorance concerning an experience or knowledge of a behaviour and the way they are measured by means of DKs and NRs. First, the specificity and diversity of the information collected in these surveys is described. Then the plurality of signification of non-responses (ignorance, embarrassment, selective amnesia) and of DKs is compared and the role of each type of response in the exploitation of survey data discussed. The article ends with a plea in favour of a qualitative approach to such responses in quantitative surveys.