Forms one polytomous super-item from each nominated group of items (its
score is the member sum; missing if any member is missing), keeps all other
items as they are, and refits the model with the same settings. The usual
treatment for item pairs flagged by residual_correlations.
Arguments
- fit
A fitted object from
rasch.- groups
A list of character vectors, each naming two or more items to combine; a single vector is also accepted.
- model
Model for the re-analysis; defaults to
"PCM", which is almost always required because subtests change the maximum scores.
Value
A new rasch fit on the combined structure, with the
combinations recorded in its notes.
Examples
set.seed(1); Np <- 500; L <- 8
d <- seq(-2, 2, length.out = L)
X <- matrix(rbinom(Np * L, 1, plogis(outer(rnorm(Np), d, "-"))), Np, L)
colnames(X) <- paste0("I", 1:L)
X[, 5] <- ifelse(runif(Np) < 0.9, X[, 4], X[, 5]) # dependent pair
fit <- rasch(X)
fit2 <- combine_items(fit, list(c("I4", "I5")))
fit2$items$item
#> [1] "I1" "I2" "I3" "I6" "I7" "I8" "I4+I5"