Should you include a “Don’t Know” option on your survey question?

Questionnaire writers construct a bridge between client objectives and a line of questioning that a respondent can understand. This is an underappreciated skill.

The best questionnaire writers empathize with respondents and think deeply about tasks respondents are asked to perform. We want to strike a balance between the level of cognitive effort required and a need to efficiently gather large amounts of data. If the cognitive effort required is too low, the data captured is not of high quality. If it is too high, respondents get fatigued and stop attending to our questions.

One of the most common decisions researchers have to make is whether or not to allow for a Don’t Know (DK) option on a question. This is often a difficult choice, and the correct answer on whether to include a DK option might be the worst possible answer: “It depends.”

Researchers have genuine disagreements about the value of a DK option. I lean strongly towards not using DK’s unless there is a clear and considered reason for doing so.

Clients pay us to get answers from respondents and to find out what they know, not what they don’t know. Pragmatically, whenever you are considering adding a DK option your first inclination should be that you perhaps have not designed the question well. If a large proportion of your respondent base will potentially choose “don’t know,” odds are high that you are not asking a good question to begin with, but there are exceptions.

If you get in a situation where you are not sure if you should include a DK option, the right thing to do is to think broadly and reconsider your goal: why are you asking the question in the first place? Here is an example which shows how the DK decision can actually be more complicated than it first appears.

We recently had a client that wanted us to ask a question similar to this: “Think about the last soft drink you consumed. Did this soft drink have any artificial ingredients?”

Our quandary was whether we should just ask this as a Yes/No question or to also give the respondent a DK option. There was some discussion back and forth, as we initially favored not including DK, but our client wanted it.

Then it dawned on us that whether or not to include DK depended on what the client wanted to get out of the question. On one hand, the client might want to truly understand if the last soft drink consumed had any artificial ingredients in it, which is ostensibly what the question asks. If this was the goal, we felt it was necessary to better educate the respondent on what an “artificial ingredient” was so they could provide an informed answer and so all respondents would be working from a common definition. Or, alternatively, we could ask for the exact brand and type of soft drink they consumed and then on the back-end code which ones have artificial ingredients and which do not, and thus get a good estimate for the client.

The other option was to realize that respondents might have their own definitions of “artificial ingredients” that may or may not match our client’s definition. Or, they may have no clue what is artificial and what is not.

In the end, we decided to use the DK option in this case because understanding how many people are ignorant to artificial ingredients fit well with our objectives. When we pressed the client, we learned that they wanted to document this ambiguity. If a third of consumers don’t know whether or not their soft drinks have artificial ingredients in them, this would be useful information for our client to know.

This is a good example on how a seemingly simple question can have a lot of thinking behind it and how it is important to contextualize this reasoning when reporting results. In this case, we are not really measuring whether people are drinking soft drinks with artificial ingredients. We are measuring what they think they are doing, which is not the same thing and likely more relevant from a marketing point-of-view.

There are other times when a DK option makes sense to include. For instance, some researchers will conflate the lack of an option (a DK response) with a neutral opinion and these are not the same thing. For example, we could be asking “how would you rate the job Joe Biden is doing as President?” Someone who answers in the middle of the response scale likely has a considered, neutral opinion of Joe Biden. Someone answering DK has not considered the issue and should not be assumed to have a neutral opinion of the president. This is another case where it might make sense to use DK.

However, there are probably more times when including a DK option is a result of lazy questionnaire design than any deep thought regarding objectives. In practice, I have found that it tends to be clients who are inexperienced in market research that press hardest to include DK options.

There are at least a couple of serious problems with including DK options on questionnaires. The first is “satisficing” – which is a tendency respondents have to not place a lot of effort on responding and instead choose the option that requires the least cognitive effort. The DK option encourages satisficing. A DK option also allows respondents to disengage with the survey and can lead to inattention on subsequent items.

DK responses create difficulties when analyzing data. We like to look at questions on a common base of respondents, and that becomes hard to comprehend when respondents choose DK on some questions but not others. Including DK makes it harder to compare results across questions. DK options also limit the ability to use multivariate statistics, as a DK response does not fit neatly on a scale.

Critics would say that researchers should not force respondents to express and opinion they do not have and therefore should provide DK options. I would counter by saying that if you expect a substantial amount of people to not have an opinion, odds are high you should reframe the question and ask them about something they do know about. It is usually (but not always) the case that we want to find out more about what people know than what they don’t know.

“Don’t know” can be a plausible response. But, more often than not, even when it is a plausible response if we feel a lot of people will choose it, we should reconsider why we are asking the question. Yes, we don’t want to force people to express an option they don’t have. But rather than include DK, it is better to rewrite a question to be more inclusive of everybody.

As an extreme example, here is a scenario that shows how a DK can be designed out of a question:

We might start with a question the client provides us: “How many minutes does your child spend doing homework on a typical night?” For this question, it wouldn’t take much pretesting to realize that many parents don’t really know the answer to this, so our initial reaction might be to include a DK option. If we don’t, parents may give an uninformed answer.

However, upon further thought, we should realize that we may not really care about how many minutes the child spends on homework and we don’t really need to know whether the parent knows this precisely or not. Thinking even deeper, some kids are much more efficient in their homework time than others, so measuring quantity isn’t really what we want at all. What we really want to know is, is the child’s homework level appropriate and effective from the parent’s perspective?

This probing may lead us down a road to consider better questions, such as “in your opinion, does your child have too much, too little, or about the right amount of homework?” or “does the time your child spends on homework help enhance his/her understanding of the material?” This is another case when thinking more about why we are asking the question tends to result in better questions being posed.

This sort of scenario happens a lot when we start out thinking we want to ask about a behavior, when what we really want to do is ask about an attitude.

The academic research on this topic is fairly inconclusive and sometimes contradictory. I think that is because academic researchers don’t consider the most basic question, which is whether or not including DK will better serve the client’s needs. There are times that understanding that respondents don’t know is useful. But, in my experience, more often than not if a lot of respondents choose DK it means that the question wasn’t designed well. 

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