This paper outlines a method of determining the achieved relative strength (ARS) of an intervention by calculating the extent to which core intervention components are present in both the treatment and control condition. Using data from several examples the authors demonstrate how to calculate ARS indices, and subsequently how to incorporate them into analyses of intervention effectiveness. The motivation intervention examples demonstrate consistency across three different methods of calculating achieved relative strength, and how treatment fidelity diffuses when moving from more controlled to less controlled settings. The Reading First example demonstrates the importance of assessing core intervention components in control conditions as the average level of implementation of Reading First components in control conditions was quite high, thus diminishing the achieved relative strength of the intervention.