CTT (version 2.3.3)

spearman.brown: Functions for Spearman-Brown "Prophecy" Formula

Description

This function calculates either a predicted reliability for a measure given the original reliability and a new test length, or the function calculates the required test length to achieve a desired level of reliability.

Usage

spearman.brown(r.xx, input = 2, n.or.r = "n")

Arguments

r.xx

The original relibility

input

The new test length or a desired level of reliability, depending on n.or.r

n.or.r

If n.or.r="n", the function will return a new reliability; if n.or.r="r", the function will return the factor by which the test length must change to achieve a desired level of reliability.

Author

John Willse, Zhan Shu

Details

If n.or.r="n", the function will return a new reliability and input should be the factor by which the test length is to be changed. If n.or.r="r", the function will return the factor by which the test length must change to achieve a desired level of reliability (provided in input).

References

Spearman, C. (1910). Correlation calculated with faulty data. British Journal of Psychology, 3, 271-295.

Brown,W. (1910). Some experimental results in the correlation of mental abilities. British Journal of Psychology, 3, 296-322.

Examples

# old relibility is 0.6, if the measure is lengthened 
# by a factor of 2, the relibility of new test is:
spearman.brown(0.6,2,"n")

# old relibility is 0.5, if we want a new measure to 
# be 0.8, the new test length is:
spearman.brown(0.5, 0.8, "r")