Plot an item characteristic curve — plot

Draws the model expected-score curve with observed class-interval means overlaid. Several items may be drawn together; their expected scores are then expressed as proportions of their maximum scores. With group supplied, observed means are drawn separately per group, the conventional graphical DIF display. For an MFRM fit, a single item may be named; its observed item-by-facet response cells are aligned by their fitted facet and interaction shifts before the class-interval means are formed.

Usage

plot_icc(
  fit,
  item,
  group = NULL,
  n_groups = NULL,
  grid = NULL,
  observed = TRUE
)

Arguments

fit: A fitted object from rasch.
item: One or more item names or column indices. Up to eight items may be overlaid. A group overlay requires a single item.
group: Optional person grouping vector, or one or more names of factors nominated in the fit, for a DIF overlay; several names give the factor-combination cells (the factorial display).
n_groups: Number of class intervals for the observed means; by default the fit's own count, or – with a group overlay – a count adapted to keep the smallest group's interval cells adequately filled.
grid: Logit grid over which to draw the model curve.
observed: Whether to add observed class-interval means.

Value

Called for its plotting side effect; invisibly NULL.

Examples

set.seed(1)
d <- seq(-2, 2, length.out = 6)
X <- matrix(rbinom(400 * 6, 1, plogis(outer(rnorm(400), d, "-"))), 400, 6)
colnames(X) <- sprintf("I%02d", 1:6)
plot_icc(rasch(X), "I03")