In interview surveys, usually a 'do-not-know’-option is not explicitly offered to a respondent, but interviewers can accept it. It is good general practice to train interviewers in using a probe after an initial 'do-not-know’ to reduce item-nonresponse.
In web surveys designers are hesitant to offer an explicit do-not-know option for fear of encouraging respondents to choose this option as a quick answer. One the other hand, not accepting do-not-know, and issuing an error message insisting on an answer may lead to either irritation and more break-offs or to guessing and less valid answers.
In this study, the interactivity of the web is used to emulate friendly interviewer probing behaviour. A high quality, probability-based Internet panel (LISS-panel, Centerdata) was used for data collection. The questionnaire contained a series of questions, which in previous self-administered (mail and web) surveys showed a high percentage of item-nonresponse. A two by two experimental design was used: (1) explicit offering of do-not-know vs. no do-not-know option, and (2) directly accepting a do-not-know vs. only accepting it after a friendly probe. As baseline for comparison a fifth condition was added with the 'standard’ web option: an error message with no acceptance of continuation without an answer.
The number of resulting do-not-know answers in each condition are evaluated and compared with the results of a telephone survey on the same topic.