The purpose of this paper is to examine longitudinal quantitative and qualitative data to uncover how an additive effect of professional development influences student outcomes. The program evaluation of a large science grant initiative outlined the delivery of a series of PD opportunities to a select number of educators throughout a large urban city school district in upstate New York. Student scores on a state science exam were analyzed not only by whether or not the student's instructor received professional development from the grant, but also by whether students received instruction from a series of educators with grant training over the three years of the grant initiative. Qualitative data, in the form of interviews and classroom observations, provided insight for future program evaluation. The authors argue that the pattern of results affirm that professional development positively influences student performance, and informs discussion for implications of professional development duration and intensity.#Science