I have this assignment of sorts to read an often-cited article by Jon Krosnick and some colleagues titled, "The Impact of 'No Opinion' Response Options on Data Quality," Public Opinion Quarterly, 66:371-403. This is quite timely as I have just finished a bit of empirical research with some colleagues that cites this article, although I confess I have not read it in several years. The research was a Web-based experiment in which we tried to assess the impact of offering a DK option in a Web survey. This is of special interest because so much of Web is about transitioning phone studies to Web and understanding some of the differences one sees. Interviewers seldom read a DK option to respondents. Rather, they hold them in their pocket and use them as a sort of last resort. Online you need to decide whether to display the DK option or not, but when you do it's not unusual to get a significant increase in its use. Hardly surprising. The key research question is whether you get an overall different distribution of substantive responses depending on whether you offer a DK option. In our study (presented by Mick Couper at the General Online Research Conference in Vienna), we got significantly higher rates of nonresponse (i.e., more frequent selection of DK rather than simply skipping the question) when we presented it on the screen. Differences in the distribution of response items question by question were difficult to detect. So the takeaway here is that not presenting the DK reduces nonresponse and does not seem to lead to a lot of guessing and nonsensical answers that destabilize the distributions.
Krosnick and his colleagues come to essentially the same conclusion. There is a body of research (see, for example, Philip Converse, "Attitudes and Non-Attitudes: Continuation of a Dialogue, " in The Quantitative Analysis of Social Problems, ed. Edward Tufte, 1970) that argues sometimes people really don't have a "preconsolidated opinion" and you can't force them to come up with one. So to keep people from guessing or answering randomly, it's best to offer a DK. Krosnick and his colleagues argue that selecting the DK is a form of satisficing that is most prevalent among respondents with less formal education, in self-administered situations, and toward the end of the questionnaire. In other words, it's a sign that respondents are not willing to put forth the cognitive energy to give a thoughtful answer, so they jump at the chance the DK offers. Take away the option and these people mostly will give valid answers and you will reduce your nonresponse.
Of course, taking away the DK option will not drive nonresponse to zero. Survey research, like life, is full of compromise. Unfortunately, due to a design flaw introduced by yours truly our research could shed no light on what happens if you don't offer a DK and don't let respondents just skip the question. On many commercial Web surveys there is no DK and an answer is required, which may produce a different result. That's a variation we need to test, but at least in this experiment our results were not terribly different from what Krosnick describes.