Sandwich covariance for a mixed-subjects fit — vcov_mixed

Estimates the full sandwich covariance matrix for item parameters from the fixed-posterior expected-count estimating equations. The parameter order is all discriminations followed by all intercepts, matching fit$par.

Usage

vcov_mixed_subjects(object, ridge = 1e-08, ...)

Arguments

object: A fitted object returned by fit_mixed_subjects() or fit_mixed_subjects_from_quadrature() with response matrices and posterior weights available in its quadrature summaries.
ridge: Small ridge value used when inverting the Hessian.
...: Unused; included for method compatibility.

Value

A covariance matrix with attributes bread and meat.

Examples

set.seed(1)
pars <- data.frame(a = c(1, 1.2, 0.9), d = c(0, -0.5, 0.3))
observed <- simulate_2pl(rnorm(40), pars)
fit <- fit_mixed_subjects(
  observed, observed, simulate_2pl(rnorm(80), pars),
  lambda = 0.5, initial_pars = pars, n_quad = 7
)
dim(vcov_mixed_subjects(fit))
#> [1] 6 6