Abstract
In the validation of tests used for selection purposes, the obtained validity coefficients are invariably underestimates of the true validities, due to explicit and implicit selection in respect of the relevant variables. Both explicit and implicit selection leads to restriction of range of the relevant variables, and this in turn reduces the obtained validities. A formal proof for this is given. A number of researchers have developed formulae for correcting sample validities in order to get better estimates of the true validities (Pearson, 1903; Thorndike, 1949; Gulliksen, 1950; Rydberg, 1962 and Lord & Novick, 1968). It is, however, virtually impossible to obtain a complete view of the problem of restriction of range in this way. In the present paper a different approach has been followed: Population correlations have been computed for various degrees of truncation of the explicit selection variable. This has been done for population correlations ranging from 0.10 to 0,99. A graphical display, indicating the shrinkage of the population correlations for various truncation ratios, has been prepared.