Abstract
<p>Previous evaluations of school textbooks have predominantly focused on aggregate overall averages, thereby overlooking critical intra-textbook heterogeneity. This study aimed to identify latent profiles of cognitive involvement and assess the instability of instructional design within Iranian senior high school history textbooks. Utilizing a secondary data analysis of datasets derived from the Comparative Adjudication Protocol (CAP)—encompassing 61 lessons across 4 core textbooks—cognitive involvement indices for text, images, and questions were calculated using the William Romey (1968) method. To quantify internal heterogeneity, the Coefficient of Variation (CV) and Levene’s test were utilized, while K-means clustering with Euclidean distance (Hair et al., 2010) was employed to uncover latent profiles.The findings revealed that the History 3 textbook exhibited the highest instructional irregularity ("CV"=144.50% in the question component), whereas the Contemporary History of Iran textbook possessed the most uniform structure ("CV"=81.58%). Levene’s test confirmed a statistically significant difference in variances across the textbooks specifically within the question component (p&lt;0.01). Furthermore, cluster analysis uncovered four distinct structural profiles, revealing that a meager 11.5% of the analyzed lessons fell into a structurally "balanced" cluster. These findings demonstrate that internal content dispersion, particularly in higher grade levels, poses a more severe pedagogical challenge than low average involvement. Consequently, future curricular interventions must prioritize variance reduction over a simple inflation of baseline averages.</p>