R package sirt. Supplementary item response theory models to complement existing functions in R, including multidimensional compensatory and noncompensatory IRT models, MCMC for hierarchical IRT models and testlet models, NOHARM, Rasch copula model, faceted and hierarchical rater models, ordinal IRT model (ISOP), DETECT statistic, local structural equation modeling (LSEM), mean and covariance structure modelling for multivariate normally distributed data.