Hi Isa,

If you have pre and post data, I would suggest estimating the growth in a single IRT model. One way is to create a wide file, load time 1 items onto one latent, and time 2 items onto a second latent, and anchor the difficulties of the items to be equal. The growth is then a function of the theta (theta2 - theta1 = growth). If you want to analyse the growth (e.g., regress covariates on the growth) you could choose a slightly different parameterisation, or you could then take PVs out of ConQuest and and fit whatever model (ANCOVA, mixed effects etc) you like in another stats package.

If you want to do it all in ConQuest, I would encourage you to read this article:

Wilson, M., Zheng, X., & McGuire, L. (2012). Formulating latent growth using an explanatory item response model approach. Journal of Applied Measurement, 13(1), 1.

I ran your model, and I think the difference between PV and WLE reliabilities is as described above. You have:

- Dim 1 and 2 have items that are very easy

- some cases have no observations on a dimension and therefore a WLE cannot be calculated

- generally few items per dimension (see Spearman-Brown prophecy formula to think about what the maximum reliability you might expect is)