Which type of assessment should be administered to determine why a 3rd grader is not fluent?

The choice of diagnostic assessments is particularly suitable for understanding why a 3rd grader is not fluent in reading. Diagnostic assessments are specifically designed to identify students' strengths and weaknesses, pinpointing specific areas of difficulty. These assessments delve deeper into the underlying skills that contribute to fluency, such as phonemic awareness, decoding, and vocabulary knowledge. By administering a diagnostic assessment, educators can gather detailed information about the child's reading abilities, understand the root causes of their lack of fluency, and tailor instruction accordingly to address those specific needs.

In contrast, formative assessments focus on monitoring student learning during the instructional process and are used to inform teaching strategies rather than diagnose issues. Summative assessments evaluate student learning at the end of an instructional unit and wouldn’t provide the detailed insights needed for addressing fluency issues. Peer assessments involve students assessing each other, which may not yield the diagnostic information necessary to identify the core reasons behind a student's lack of fluency. Thus, diagnostic assessments are the most effective tool for uncovering the specific challenges a student faces in achieving reading fluency.