The classical companion table conventionally reported alongside a Rasch analysis (Andrich and Marais 2019, chs. 3-5), on complete cases only: per item the facility (mean score over maximum), the item-total and corrected item-rest correlations, the discrimination index DI = PRU - PRL (mean proportion-of-maximum in the upper third of total scores minus the lower third), and alpha if the item is deleted; plus coefficient alpha, the raw-score mean, SD, and the classical standard error of measurement \(s\sqrt{1 - \alpha}\), which unlike the Rasch SE is one value for all persons.
Arguments
- fit
A fitted object from
rasch.
Value
A list of class "rasch_ctt": the per-item table
(item, n, min, max, facility,
item_total, item_rest, di, alpha_drop), and
the scalars
alpha, n (complete cases), mean, sd, and
sem.
Examples
set.seed(1)
d <- seq(-2, 2, length.out = 8)
X <- matrix(rbinom(400 * 8, 1, plogis(outer(rnorm(400), d, "-"))), 400, 8)
colnames(X) <- paste0("I", 1:8)
ctt_table(rasch(X))
#> Traditional statistics (available cases; item n 400-400; 400 complete)
#> Raw score mean 4.08, SD 1.71 (complete responders); alpha 0.529; SEM 1.17
#> item n min max facility item_total item_rest di alpha_drop
#> I1 400 0 1 0.848 0.370 0.169 0.298 0.519
#> I2 400 0 1 0.787 0.443 0.221 0.373 0.504
#> I3 400 0 1 0.672 0.479 0.226 0.484 0.504
#> I4 400 0 1 0.575 0.526 0.267 0.589 0.488
#> I5 400 0 1 0.432 0.579 0.333 0.663 0.460
#> I6 400 0 1 0.338 0.495 0.243 0.497 0.497
#> I7 400 0 1 0.250 0.533 0.313 0.527 0.472
#> I8 400 0 1 0.175 0.407 0.198 0.316 0.511