Hi everyone,

I have what I think is a relatively straightforward IRT model that I’m having difficulty getting to converge. I identify the model by ordering two parameters zeta_j to avoid reflection invariance, and by fixing the scale by placing a standard normal prior on zeta. I nevertheless end up with a small number of chains that don’t converge (maybe 1 out of 8 chains). I’ve tried a bunch of strategies, including setting redundant identification constraints, dropping the constant lamda and thus letting the zeta_j and theta_i scale themselves, setting hard constraints by fixing two zeta_j to constants (e.g. -1 and +1), and tightening up priors (maybe not enough?). When I initialize the parameters in reasonable ways, I don’t have any problems however. I’m alright with fitting the model with just these initializations, but if there’s a better way, it’d be great to know! Thanks!

IRT.stan (1.4 KB)