WrightMap (version 1.4)

extract.deltas: Extract Master's Delta parameters from a TAM model.

Description

This function takes as its input a TAM object. It adds reads the TAM item parameters and organizes them into a matrix that can be used as input in the CCCfit function.

Usage

extract.deltas(tamObject)

Value

A matrix in which each row is an item and each column is a step

Arguments

tamObject

TAM object containing the results of a a Rasch model or Partial Credit model.

Author

David Torres Irribarra

Details

This function organizes the item parameter results into a matrix where each row is contains the parameters associated with an item and each columns is contains the parameters associated with a specific step (score 0 vs score 1, score 1 vs score 2, etc.). The resulting matrix will have as many rows as items and as many columns as the maximum number of steps among the items.

References

Masters, G. N. (1982). A Rasch model for partial credit scoring. Psychometrika, 47(2), 149-174.

See Also

CCCfit make.thresholds

Examples

##---- Should be DIRECTLY executable !! ----
##-- ==>  Define data, use random,
##--	or do  help(data=index)  for the standard data sets.

## The function is currently defined as
function (tamObject) 
{
    delta.long <- tamObject$xsi
    n.deltas <- apply(tamObject$B, 1, max)
    delta.mat <- matrix(NA, nrow = length(n.deltas), ncol = max(n.deltas))
    matCoords.row <- rep(1:length(n.deltas), n.deltas)
    matCoords.col <- c()
    for (i in 1:length(n.deltas)) {
        for (j in 1:n.deltas[i]) {
            matCoords.col <- c(matCoords.col, j)
        }
    }
    delta.long$matCoords.row <- matCoords.row
    delta.long$matCoords.col <- matCoords.col
    for (k in 1:nrow(delta.long)) {
        delta.mat[delta.long$matCoords.row[k], delta.long$matCoords.col[k]] <- delta.long$xsi[k]
    }
    delta.mat
  }