Replaces each nominated item with one item per estimable level of a person
factor, each carrying that level's responses only (other levels missing).
A level must contain every score from zero to the fitted item maximum;
levels missing a score are omitted and recorded in the notes. The split
is refused if calibration subsequently merges a category lacking conditional
information within a retained group. Every retained
group then receives its own item location, which resolves the invariance
violation flagged by dif_anova; the distance between the
split locations estimates the DIF size. The model is refitted with the
same settings.
Arguments
- fit
A fitted object from
rasch.- items
Character vector naming the item(s) to split.
- by
The name of a person factor nominated in the fit, or a grouping vector with one entry per person.
Value
A new rasch fit in which each split item appears as
"item (level)", with the splits recorded in its notes. Person and
item estimates are recalculated with the fitted grouping, keyed scoring,
anchors on unchanged items, PCM constraints and optimisation controls.
An anchored item cannot be split.
See also
resolve_dif, which applies this iteratively;
drop_items, which removes an item rather than resolving
it; and dif_anova, which identifies the items to split.
Examples
set.seed(1); n <- 600
d <- seq(-2, 2, length.out = 8); g <- rep(c("a", "b"), each = n / 2)
sh <- matrix(0, n, 8); sh[g == "b", 3] <- 1
X <- matrix(rbinom(n * 8, 1, plogis(outer(rnorm(n), d, "-") - sh)), n, 8)
colnames(X) <- paste0("I", 1:8)
fit <- rasch(data.frame(X, grp = g), factors = "grp")
fit2 <- split_items(fit, "I3", by = "grp")
fit2$items$item
#> [1] "I1" "I2" "I4" "I5" "I6" "I7" "I8" "I3 (a)"
#> [9] "I3 (b)"