psychometric (version 2.4)

discrim: Item Discrimination

Description

Discrimination of an item is the ability for a specific item to distinguish among upper and lower ability individuals on a test

Usage

discrim(x)

Value

Discrimination index for each item in the data.frame or matrix analyzed.

Arguments

x

matrix or data.frame of items to be examined. Rows represent persons, Columns represent items

Author

Thomas D. Fletcher

Details

The function takes data on individuals and their test scores and computes a total score to separate high and low ordered individuals. The upper and lower groups are defined as the top and bottom 1/3 of the total. Discrimination is then computed and returned for each item using the formula:
(number correct in the upper group - number correct in the lower group ) / size of each group

References

Allen, M. J. & Yen, W. M. (1979). Introduction to measurement theory. Monterey, CA: Brooks/Cole.

See Also

item.exam

Examples

# see item.exam
# Scores on a test for 12 individuals
# 1 = correct
item1 <- c(1,1,1,0,1,1,1,1,1,1,0,1)
item2 <- c(1,0,1,1,1,1,1,1,1,1,1,0)
item3 <- c(1,1,1,1,1,1,1,1,1,1,1,1)
item4 <- c(0,1,0,1,0,1,0,1,1,1,1,1)
item5 <- c(0,0,0,0,1,0,0,1,1,1,1,1)
item6 <- c(0,0,0,0,0,0,1,0,0,1,1,1)
item7 <- c(0,0,0,0,0,0,0,0,1,0,0,0)
exam <- cbind(item1, item2, item3, item4, item5, item6, item7)
discrim(exam)