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

## 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

 `1` ```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.

## 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).

## Author(s)

John Willse, Zhan Shu

## 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

 ```1 2 3 4 5 6 7``` ```# 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") ```

CTT documentation built on May 2, 2019, 1:08 p.m.