The purpose of this chapter is to describe initial validity evidence gathered to examine the trustworthiness and meaningfulness of the claims made about a progress monitoring assessment system designed to measure middle school students’ algebra readiness. The Algebra Readiness Progress Monitoring (ARPM) assessment is intended to help teachers determine if students are progressing toward algebra readiness at an appropriate rate and focuses on students’ numeric relational reasoning skills. This chapter will present initial validity evidence gathered to evaluate whether these items sufficiently elicit students’ reasoning skills. Specifically, we focus on evidence examining students’ response processes. Evidence is presented to begin to substantiate two primary claims: (1) items designed using a systematically developed template elicit the same reasoning strategy, and (2) items may be solved using numeric relational reasoning. The evidence from these studies can be used as first-round evidence to substantiate or refute the claims that the ARPM system is useful for drawing conclusions about students’ numeric relational reasoning. Information about additional methods of collecting students’ response processes are also discussed, including the use of interviews and eye-tracking data.