itm function

Extract Item Parameters

itm is a generic function which extracts a data frame containing the item parameters of two forms being equated in the original scale and item parameters of the first form converted in the scale of the second form.

itm(x, ...)

## S3 method for class 'eqc'
itm(x, ...)

## S3 method for class 'eqclist'
itm(x, link, ...)

## S3 method for class 'ceqc'
itm(x, ...)

## S3 method for class 'ceqclist'
itm(x, path, ...)

## S3 method for class 'meqc'
itm(x, link, bistype = NULL, ...)

Arguments

• x: object of the class eqc returned by function direc

  or of the class eqclist returned by function alldirec

  or of the class ceqc and ceqclist returned by function chainec

  or of the class meqc return by function bisectorec.

• link: a character string with the names of the two forms being linked separated by a dot (e.g. "test1.test3").

• path: a character string with the names of the forms that constitute the path separated by a dot (e.g. "test1.test2.test3").

• bistype: bisector type. It should be weighted or unweighted. It could be NULL if only one type was computed by function bisectorec.

• ...: further arguments passed to or from other methods.

Returns

A data frame containing item names (Item), item parameters of the first form (e.g. test1), item parameters of the last form (e.g. test3), and item parameters of the first form converted in the scale of the last form (e.g. test1.as.test3).

Author(s)

Michela Battauz

See Also

convert, alldirec, bisectorec, chainec, direc, modIRT

Examples

# the following code uses item parameter estimates previously obtained
# for examples that start from item response data see function modIRT

# two-parameter logistic model
data(est2pl)
test<-paste("test", 1:5, sep = "")
mod2pl <- modIRT(coef = est2pl$coef, var = est2pl$var, names = test, display = FALSE)
# direct equating coefficients between forms 1 and 2 using the Haebara method
l12 <- direc(mods = mod2pl, which = c(1,2), method = "Haebara")
# all direct equating coefficients using the Haebara method
direclist2pl <- alldirec(mods = mod2pl, method = "Haebara")
# compute all chain equating coefficients of length 3
chainec3 <- chainec(r = 3, direclist = direclist2pl)
# compute chain equating coefficients for path 1,2,3,4
pth1 <- paste("test", 1:4, sep = "")
chainec1 <- chainec(direclist = direclist2pl, pths = pth1)
# compute chain equating coefficients for path 1,5,4
pth2 <- c(paste("test", c(1,5,4), sep = ""))
chainec2 <- chainec(direclist = direclist2pl, pths = pth2)
# create a list of objects of class ceqc
ecall <- c(chainec1, chainec2)
# compute bisector and weighted bisector coefficients
allec <- bisectorec(ecall = ecall, weighted = TRUE, unweighted = TRUE)

# extract item parameters
itm(l12)
itm(direclist2pl, link = "test1.test2")
itm(chainec3, path = "test1.test2.test3")
itm(allec, bistype = "weighted")