I am trying to estimate IRT a (item discrimination) and b (item difficulty) parameters from the probability of correct response (=p+ values) conditioned on each of the 8 theta points. Someone told me that I have to use logit function to get the "a" and "b" parameters. So I ran the proc Genmod procedure as shown below, but the result looks not right.
Data temp;
input theta p;
card;
-3.0 .001
-2.5 .015
---
2.5 .950
3.0 .998
;;;;
run;
proc genmod data=temp;
model p=theta / link=logit, dist=poi;
run;
Would you let me know what SAS function I have to use to get the correct "a" and "b" parameters?
Thanks!