I would describe a model (and solve with the
stan software) a process like this:
for i = 1...N consumers
- the i-th consumer is allowed to choose an item out of 7. 6 items remain.
- the same consumer chooses another item out of the 6 remaining items.
I collected the N pairs of choices and I would like to infer if there are some preferences. The first step is a multinomial model. The second is a multinomial process conditioned by the first choice. I really struggle to model the log probability function and code it in stan. Any help?