[Limdep Nlogit List] Choice of data to use

[medard kakuru]

Dear Members,
Please advise on which data set to use.
My study is about estimating willingness to pay (wtp) for organic solid
waste fertilizer. I used a choice experiment (CE) with two alternatives and
an opt-out/no choice. During data collection, to ensure random sampling, I
interviewed farmers who had ever used any fertilizer and those who have
never. Those who said never were not given the choice sets to elicit wtp.

The first set of results is using the entire sample (both ever and never
used fertilizers). This implies that there is missing CE data for  those
who have never. I circumvent this by assuming that these farmers, if they
had been given the choice sets, would opt-out/ make no choice. I get good
results, even the price parameter is negative and significant.

The second set of results is using only a subset of only those given the
choice sets (ever used fertilizers). The price parameter is positive and
significant, contrary to economic theory.

Please, advise on which dataset I should consider for my analysis.

PS. I estimated mixed logit models in utility space and wtp space

Regards,

Medard