In reading intervention research, implementation fidelity is assumed to be positively related to student outcomes, but the methods used to measure fidelity are often treated as an afterthought. Fidelity has been conceptualized and measured in many different ways, suggesting a lack of construct validity. One aspect of construct validity is the fidelity index of a measure. This methodological case study examined how different decisions in fidelity indices influence relative rank ordering of individuals on the construct of interest and influence our perception of the relation between the construct and intervention outcomes. Data for this study came from a large state-funded project to implement multi-tiered systems of support for early reading instruction. Analyses were conducted to determine whether the different fidelity indices are stable in relative rank ordering participants and if fidelity indices of dosage and adherence data influence researcher decisions on model building within a multilevel modeling framework. Results indicated that the fidelity indices resulted in different relations to outcomes with the most commonly used fidelity indices for both dosage and adherence being the worst performing. The choice of index to use should receive considerable thought during the design phase of an intervention study.