My data is 2688 by 200, with 0/1 coded. I am trying to estimate both parameters and ability(theta) using 2PL logistic model.

First I use the following code to get parameter estimate:

fit2PL = ltm(dat ~ z1, IRT.param = TRUE)

Then use the following to get theta estimate

theta<-ltm::factor.scores(fit2PL)

Now I want to extract zl column (i.e., theta estimate) from theta, the following code is not working. please help

theta1<-theta$zl

thank you!