  A central goal of the Learning Assistant (LA) model is to improve students'
learning of science through the transformation of instructor practices. There
is minimal existing research on the impact of college physics instructor
experiences on their effectiveness. To investigate the association between
college introductory physics instructors' experiences with and without LAs and
student learning, we drew on data from the Learning About STEM Student Outcomes
(LASSO) database. The LASSO database provided us with student-level data
(concept inventory scores and demographic data) for 4,365 students and
course-level data (instructor experience and course features) for the students'
93 mechanics courses. We performed Hierarchical Multiple Imputation to impute
missing data and Hierarchical Linear Modeling to nest students within courses
when modeling the associations between instructor experience and student
learning. Our models predict that instructors' effectiveness decreases as they
gain experience teaching without LAs. However, LA supported environments appear
to remediate this decline in effectiveness as instructor effectiveness is
maintained while they gain experience teaching with LAs.
