Hi team,

I am designing a DCE with 3 alternatives (follow-up A, follow-up B, and None). In my study, ‘none’ indicates that respondents do not prefer any of the provided options (A and B), but it does not necessarily mean that they do not prefer other available follow-up at all.

I used the below formula to estimate minimum number of choice tasks:

S_min = L/(A-1)

Where S_min is the minimum number of choice tasks, L is the total number of levels of all attributes, and A is the number of alternatives.

Should the above formula include ‘none’ option in the alternatives? In other words, should it be A=2 or A=3? I am uncertain whether selecting ‘none’ contributes any information on trade-offs that respondents are willing to make among attribute levels.

Thank you.

Kind regards,

Pakhi