Before an assessment is used to make decisions, the validity of the intended interpretation must be evaluated. The purpose of this paper is to describe how the argument-based approach and an interpretation/use argument (IUA) (Kane, 2013) were used to validate the interpretations made from the new Early Grade Mathematics Assessment (EGMA) Relational Reasoning subtask. Three assumptions were identified on the IUA and evidence collected for each. Overall, the clarity, coherence, completeness of the IUA and the plausibility of the assumptions indicate that there is moderate evidence to support the claim that the score on the EGMA Relational Reasoning subtask can be interpreted to determine overall student performance in relational reasoning.