This paper examines normalized gains and preinstruction scores on the force concept inventory (FCI) for students in interactive engagement courses in introductory mechanics at four universities and found a significant, positive correlation for three of them. We also examined class average FCI scores of 2948 students in 38 interactive engagement classes, 31 of which were from the same four universities and 7 of which came from 3 other schools. We found a significant, positive correlation between class average normalized FCI gains and class average preinstruction scores. To probe this correlation, we administered Lawson's classroom test of scientific reasoning to 65 students and found a significant, positive correlation between these students' normalized FCI gains and their Lawson test scores. This correlation is even stronger than the correlation between FCI gains and preinstruction FCI scores. Our study demonstrates that differences in student populations are important when comparing normalized gains in different interactive engagement classes. We suggest using the Lawson test along with the FCI to measure the effectiveness of alternative interactive engagement strategies.