At grades 8 and 12, the 1992 National Assessment of Educational Progress (NAEP) reading assessment contained a small number of 50-minute blocks in addition to the usual 25-minute blocks. To determine whether to incorporate the 50-minute blocks into the operational scaling, this study sought to determine whether the longer blocks measured a different construct from that assessed by the 25-minute blocks. Structural equation modeling tested the hypothesis that the structural parameters relating reading ability to demographic variables do not differ across block type. A multiple group analysis, where type of block (25-minute or 50-minute) defined the two groups, was used. The null hypothesis was that the two types of blocks measure the same trait but could differ in observed mean and variance. Results of the main analysis did not reject the hypothesis of invariant structural parameters, and so the 50-minute blocks were not incorporated into the 1992 NAEP scales. Sensitivity analyses indicated that this conclusion was moderately robust to assumptions made about missing data for items that were not reached. Analyses using other measures of fit yielded similar results, although the magnitude of chi-square statistics was affected by the fit measure chosen. (Contains 6 tables and 13 references.) (Author/SLD)