ABSTRACT
The equation for the relationship between test length and test reliability will be developed from the usual formula for the correlation between any two sums. No assumptions will be used in the derivation until the last step. There it will be assumed that the variance of the unit test can be taken as a fair approximation to the average variance of all the unit parallel forms, and the reliability of the unit test times its variance can be taken as a reasonable approximation to the average covariance among all the unit parallel forms.
