A project was carried out to determine the degree of content homogeneity that a test item pool must have in order to accomplish successful Rasch calibration. Mathematics item pools were administered to upper elementary children. The items were analyzed under two conditions, with items organized into separate subtests and as a global mathematics test. Results indicate that satisfactory item calibration can be carried out with global mathematics tests, avoiding the necessity of organizing items into separate subtests. The degree to which calibration is identical under the two conditions is a topic for further study. (Author)