.onLoad                 Registers S3 methods at load time: - print for
                        class "rdif" - plot for class "rdif" - summary
                        for class "rdif"
a_fun                   The R-DIF scaling function for item slopes.
bsq_weight              The bi-square weight function.
d_fun                   The R-DIF scaling functions for item intercepts
                        / thresholds.
delta_test              Wald test of differential test functioning.
dif_test                Wald tests of differential item functioning
                        (DIF).
format_pars             Helper function used to format parameters
                        estimates
get_lavaan_pars         Extract item parameter estimates and their
                        covariance matrix from 'lavaan'.
get_mirt_pars           Extract item parameter estimates and their
                        covariance matrix from 'mirt'.
get_model_parms         Extract and format item parameter estimates and
                        their covariance matrix
get_starts              Compute starting values for 'rdif'.
grad_a                  The gradient matrix of 'a_fun'.
grad_d                  The gradient matrix of 'd_fun'.
lts                     The least trimmed squares (LTS) estimate of
                        location
psi                     The bi-square psi function.
psi_prime               The derivative of the bi-square psi function.
rdif                    Estimate IRT scale parameters using the robust
                        DIF procedure.
rdif.eg                 Example data set for R-DIF functions.
rho                     The bi-square rho function.
rho_grid                Compute a grid of bi-square Rho values
vcov_y                  The covariance matrix of IRT scaling functions.
y_fun                   R-DIF scaling functions for item
                        intercepts/thresholds and slopes.
