This study examines the effects of implementing various research-based forms of curricula for teaching introductory physics course at the College of DuPage in Glen Ellyn, IL and the University of Maine in Orono, ME. Introductory courses at each of these institutions were modified at each of these institutions over the course of several years. For each course, baseline data were collected during a year in which at least one form of research-based curricula was used. In subsequent years, other research-based curricula were implemented in addition to the baseline treatment at the University of Maine and in place of the baseline treatment at the College of DuPage. Data collected using the Force and Motion Conceptual Evaluation and the Force Concept Inventory were analyzed using the Model Analysis method developed by Bao and Redish to determine the manner(s) in which the curricular modifications affected students' conceptual development throughout a course. The results of this analysis show that the majority of curricular changes had little overall effect on students' conceptual understanding of physics. Several exceptions to this generality are discussed.