reliability {CTT}

Function for item reliability analysis

Description

This function performs reliability analyses, providing coefficient alpha and item statistics.

Usage

reliability(items, itemal = TRUE, NA.Delete = TRUE, ml = TRUE)

Arguments

items

The scored response file with "0" (wrong) and "1" (correct) or Likert type data

itemal

If itemal=FALSE (the default) no item analyses are conducted. If itemal=TRUE, the function will provide item means, item total correlations, and alpha if item is removed.

NA.Delete

If NA.Delete=TRUE (the default) records are deleted listwise if there are missing responses. If NA.Delete=FALSE all NA values are changed to 0s.

ml

A logical variable indicating whether the biserial correlation is calculated using a formal maximum likelihood estimator (default) or an ad hoc estimator.

Details

The input files must be scored files with "0" and "1" or numeric scales (e.g., Likert Type scales). Only basic scale information is returned to the screen. Use str() to view additional statistics that are available.

Value

nItem

The number of items

nPerson

The sample size used in calculating the values

alpha

Crobach's alpha

scaleMean

Average total sum socre

scaleSD

Standard deviation of total sum score

alphaIfDeleted

Cronbach's alpha if the corresponding item were deleted

pBis

The item total correlation, with the item's contribution removed from the total

bis

The item total biserial (or polyserial) correlation, with the item's contribution removed from the total'

itemMean

Average of each item

Author(s)

John T. Willse, Zhan Shu

References

Cronbach, L. J. (1951). Coefficient alpha and the internal structure of tests. Psychometika, 16, 297-334.

See Also

itemAnalysis

Examples

# Scored input (data frame is preferred)
 x<-data.frame(matrix(c(0,0,0,0,0,
                        0,0,0,0,0,
                        0,0,0,0,1,
                        0,0,0,1,1,
                        0,0,1,1,1,
                        0,1,1,1,1,
                        1,1,1,1,1,
                        1,0,1,1,1,
                        0,0,0,1,1,
                        0,1,1,1,1),nrow=10,ncol=5,byrow=TRUE,
                        dimnames=list(c(paste("P",c(1:10),sep="")),c(paste("I",c(1:5),sep="")))))
reliability(x, itemal=TRUE)

# To see more item statisitics
str(reliability(x,itemal=TRUE))