Middle school data from the Third International Mathematics and Science Study (TIMSS) are analyzed in this study to compare achievement differences between adjacent grades. To facilitate this computer-based analysis, a Statistical Analysis System (SAS) program has been developed to transpose matrices of the item scores in more than 40 countries. The results indicate that not all TIMSS items have a higher mean score at the upper grade in each nation. Features of item construction are discussed to disentangle the issue of reflecting grade gaps in mathematics and science achievement. These analyses may help enrich understanding of other comparative studies using the TIMSS benchmark. (Contains 5 tables and 15 references.) (Author/SLD)