Letter series and number series tests, consisting of items based on identical rules, were administered in a counterbalanced design to 320 undergraduates in order to determine their comparability by testing the equality of (a) means, (b) variances, and (c) validity coefficients. Results indicated that the letter and number series tests, in which each item in one had a same-rule counterpart in the other, were not equivalent because they did not satisfy one or more of the criteria. Number series tests have higher means ( p < .001) and lower variances and covariances ( p < .01) than the corresponding letter series forms.