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Extract Item Parameters of Plackett-Luce Models

Source: R/itempar.R
itempar.PlackettLuce.Rd

Methods for itempar to extract the item parameters (worth or log-worth) from a Plackett-Luce model or tree. In the case of a tree, item parameters are extracted for each terminal node.

Usage

# S3 method for PlackettLuce
itempar(object, ref = NULL, alias = TRUE, vcov = TRUE, log = FALSE, ...)

# S3 method for pltree
itempar(object, ...)

# S3 method for PLADMM
itempar(object, ref = NULL, alias = TRUE, vcov = TRUE, log = FALSE, ...)

Arguments

object

a fitted model object as returned by PlackettLuce, pladmm, or pltree.

ref

a vector of labels or position indices of item parameters which should be used as restriction/for normalization. If NULL (the default), all items are used with a zero sum (log = TRUE) or unit sum (log = FALSE) constraint.

alias

logical. If TRUE (the default), the aliased parameter is included in the return vector (and in the variance-covariance matrix if vcov = TRUE). If FALSE, it is removed. If the restriction given in ref depends on several parameters, the first parameter of the restriction specified is (arbitrarily) chosen to be removed if alias is FALSE.

vcov

logical. If TRUE (the default), the (transformed) variance-covariance matrix of the item parameters is attached as attribute vcov. If FALSE, a NA-matrix is attached.

log

logical. Whether to return log-abilities (TRUE) or abilities (FALSE).

...

further arguments which are currently not used.

Value

An object of class "itempar", see itempar.

Examples

R <- matrix(c(1, 2, 0, 0,
              4, 1, 2, 3,
              2, 1, 1, 1,
              1, 2, 3, 0,
              2, 1, 1, 0,
              1, 0, 3, 2), nrow = 6, byrow = TRUE)
colnames(R) <- c("apple", "banana", "orange", "pear")

mod <- PlackettLuce(R)
coef(mod)
#>       apple      banana      orange        pear        tie2        tie3 
#>  0.00000000  0.25287379 -0.61350684 -0.08688475 -2.15068111 -0.79245358 

# equivalent to default coefficients, i.e. log abilities
itempar(mod, ref= 1, log = TRUE)
#> Item response item parameters (PlackettLuce):
#>    apple   banana   orange     pear 
#>  0.00000  0.25287 -0.61351 -0.08688 

# abilities, normalized so abilities for apple and pear sum to 1
itempar(mod, ref = 1:2)
#> Item response item parameters (PlackettLuce):
#>  apple banana orange   pear 
#> 0.4371 0.5629 0.2367 0.4007