Running examples in 'psychotools-Ex.R' failed
The error most likely occurred in:

> ### Name: pcmodel
> ### Title: Partial Credit Model Fitting Function
> ### Aliases: pcmodel PCModel.fit print.pcmodel summary.pcmodel
> ###   print.summary.pcmodel coef.pcmodel bread.pcmodel estfun.pcmodel
> ###   logLik.pcmodel vcov.pcmodel
> ### Keywords: regression
> 
> ### ** Examples
> 
> o <- options(digits = 4)
> 
> ## Verbal aggression data
> data("VerbalAggression", package = "psychotools")
> 
> ## Partial credit model for the other-to-blame situations
> pcm <- pcmodel(VerbalAggression$resp[, 1:12])
> summary(pcm)

Partial credit model

Item category parameters:
               Estimate Std. Error z value Pr(>|z|)
S1WantCurse-C2   0.4841         NA      NA       NA
S1DoCurse-C1    -0.2513         NA      NA       NA
S1DoCurse-C2     0.3782         NA      NA       NA
S1WantScold-C1   0.4177         NA      NA       NA
S1WantScold-C2   1.0124         NA      NA       NA
S1DoScold-C1     0.4785         NA      NA       NA
S1DoScold-C2     1.5179         NA      NA       NA
S1WantShout-C1   0.6884         NA      NA       NA
S1WantShout-C2   2.1801         NA      NA       NA
S1DoShout-C1     1.5682         NA      NA       NA
S1DoShout-C2     3.2902         NA      NA       NA
S2WantCurse-C1  -0.5966         NA      NA       NA
S2WantCurse-C2  -0.2195         NA      NA       NA
S2DoCurse-C1     0.0973         NA      NA       NA
S2DoCurse-C2     0.6992         NA      NA       NA
S2WantScold-C1   0.2555         NA      NA       NA
S2WantScold-C2   0.8907         NA      NA       NA
S2DoScold-C1     0.8441         NA      NA       NA
S2DoScold-C2     2.2998         NA      NA       NA
S2WantShout-C1   0.8718         NA      NA       NA
S2WantShout-C2   1.9785         NA      NA       NA
S2DoShout-C1     1.9050         NA      NA       NA
S2DoShout-C2     4.0650         NA      NA       NA

Log-likelihood: -2480 (df = 23) 
Number of iterations in BFGS optimization: 10 

> 
> ## visualizations
> plot(pcm, type = "profile")
> plot(pcm, type = "regions")
> plot(pcm, type = "curves")
> plot(pcm, type = "information")
> plot(pcm, type = "piplot")
> 
> ## Get data of situation 1 ('A bus fails to
> ## stop for me') and induce a null category in item 2.
> pcd <- VerbalAggression$resp[, 1:6, drop = FALSE]
> pcd[pcd[, 2] == 1, 2] <- NA
> 
> ## fit pcm to these data, comparing downcoding and keeping strategy
> pcm_va_keep  <- pcmodel(pcd, nullcats = "keep")
Warning in pcmodel(pcd, nullcats = "keep") :
  There are items with null categories (I2).
Error in solve.default(vc) : 
  system is computationally singular: reciprocal condition number = 3.21181e-164
Calls: pcmodel -> solve -> solve.default
Execution halted