Student explanations of their mathematical thinking and conclusions have become a greater part of the assessment landscape in recent years. With a sample of 71 4th-grade students at-risk for mathematics learning disabilities, we investigated the relation between student accuracy in comparing the magnitude of fractions and the quality of students' explanations of those comparisons, as well as the relation between those measures and scores on a criterion test: released fraction items from the National Assessment of Educational Progress (NAEP). We also considered the extent to which reasoning and language contribute to the prediction. Results indicated a significant, moderate correlation between accuracy and explanation quality. Commonality analyses indicated explanation quality accounts for little variance in NAEP scores beyond what is accounted for by traditional measures of magnitude understanding. Implications for instruction and assessment are discussed. [At time of submission to ERIC this article was in press with "Journal of Learning Disabilities".]