The purpose of the study was to validate a theoretical model of diagnostic, formative classroom assessment called, "Proximal Assessment for Learner Diagnosis" (PALD). To achieve its purpose, the study employed a two-stage, mixed-methods design. The study utilized multiple data sources from 11 elementary level mathematics teachers who received training in the theoretical principles underlying the model. The PALD model was derived based on the cognitive and assessment sciences literature. In the first stage of the study, a grounded theory analysis of the data yielded themes and sub-themes that were connected to reveal how diagnostic assessment practices were embedded by teachers during instruction. In the second stage, a quantitative cluster analysis was employed to validate groupings of themes that emerged from the qualitative analysis. A revised model of diagnostic classroom assessment is presented from the cross-validated findings. The findings suggested a new "theory of PALD practice" that occurs in unison with the theoretically expected practices of teachers in the originally proposed model. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]