However, high correlation by itself provides weak evidence for substitutability. Two highly correlated independent measure may have widely different correlations with our criterion. However the validities can be that different only if both tests are perfectly reliable. When two test correlate .90, the unshared variance can often be attributed to random error. Test scores may be heavily influenced by a variable that is not pertinent to your purpose.
Tests using a common method show higher intercorrelations than do different kinds of tests.
Researchers commonly cite general intelligence, g , as a cause of high correlation between tests. For a criterion unrelated to g , that irrelevant variable could produce a misleading correlation. Let's permute the set of correlations given in paragraph two, above. A test with .00 correlation with another test may be a good substitute. If the original test is correlated .44 with the criterion, the uncorrelated substitute could correlate as high as .90.