Sometimes choice questions include a 'none of these' option, as in the example below. These are generically referred to as none of these, but any wording can be used.
It is also possible to have multiple 'none of these' alternatives. E.g.,
- I would buy something else.
- I would not buy anything at all.
When generating an experimental design you first create the design without this opinion, and then just add this extra option to the choice questions (i.e., there is no special part of the experimental design process that is required).
Most people who are new to choice-based conjoint instinctively think it is a good idea to add such an option. This usually derives from two likely incorrect assumptions:
- It is seen as being a bit like having a Don’t know option in a normal questionnaire.
- The hope is that it will measure category expansion effects, allowing the resulting model to predict how changes in, say, the prices of different alternatives will result in changes in the overall market demand.
However, including this option comes with considerable costs. In particular:
- When given such an option some people may click this option as an easy alternative to reading the other options (i.e., they satisfice). If that happens, then the validity of the entire study is poor.
- You need a larger sample size if using this option. For example, if the None option is chosen half the time, then the required sample size will, else being equal, need to be twice as large.
- Often people do have none of these as an option in the real world. For example, a family must have electricity, water, and foodstuffs, so giving a 'none of these' can sacrifice ecological validity.
- Looking at the example above, when a person is choosing between the four brands, they can make this choice just by reflecting on their preferences then and there. Provided these preferences are stable over time - and often they may well be - the data will be good. However, in the real world, the decision to choose not to buy any of the alternatives will be driven by a myriad of situational factors (e.g., do they have the product at home already, how hungry they are, who they are with), and it seems unlikely that any of these will be salient at the time of filling in the questionnaire.