spearman.brown: Functions for Spearman-Brown "Prophecy" Formula
In CTT: Classical Test Theory Functions

Description Usage Arguments Details Author(s) References Examples

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.

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

`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.

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).

John Willse, Zhan Shu

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.

# 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")