I am interested in fitting a multi-dimensional IRT model for exploratory item factor analysis. The probability of getting an item correct is a function of an item intercept + slope*theta where both the slope and theta are multidimensional of order k. The problem is that for identification purposes some of the slopes need to be fixed to 0 for the first k items. For example, for item 1 the slope vector would be something like (a, 0, 0,…0) where a is a free parameter for the slope. If we think of the slopes for all the items as consisting of a j x k matrix with j = the number of items, the first k rows of that matrix would be lower triangular with zeros above the diagonal.
My question concerns the process of fixing parameters in Stan. This can be easily done by loops but I wanted to avoid loops. My question is can parameters be fixed to constant values in Stan?
Thanks for your help