Dear Mike,
I'd like to ask you about item fit statistics. I understand that to arrive at infit and outfit values if done by hand, one would first need to calculate the difference between the observed score (which is a 0 or a 1 if it's a dichotomous item) and the modeled probability.
Now, my question concerns this, in order to arrive at the modeled probability, we'd need to know the true values of item difficulty and person ability. If I'm not mistaken, when calculating the fit statistics, Facets uses the parameter estimates. Is there a correction implemented?
It's not really a big deal, I'm just interested.
Cheers
M.

Mokusmisi, unfortunately we never know the true values of anything. We can only use estimates. Facets uses the parameter estimates when computing fit statistics. These fit statistics are never exact, but they are good enough for practical purposes.

Thank you. Like I said, just out of curiosity. (I'm not a statistician anyway).