Abstract: Over the past two years, schools have made many changes in instructional delivery (largely as a result of the Pandemic). These instructional changes have dramatically increased the role of PD, as teachers seek to implement new and existing tools for maximum student benefit. While schools aggressively investigate PD solutions, proof of effectiveness has taken center stage. Schools want to know if the products and services they are purchasing “work”. This growth in interest in have driven researchers and educators to look for new and better ways to get evidence of product effectiveness, while “staying within the lines” of rigorous scientific inquiry. MLM offers the promise of strengthening the quality of evidence and processes for evaluating product and service effectiveness. MLM analyzes the relationships among multiple predictor variables and one or more outcome variables. Unlike traditional Ordinary Least Squares (OLS) models (such as ANOVA or Regression), MLM is not limited to studying individuals and can more accurately study group level variables such as teachers within schools. The MLM model confirms and extends the effectiveness finding of the [PD interventions name] for improving student reading skills and broadens those findings. The amount of variance in student reading skill growth accounted for based on participation in PD was substantially higher for the MLM model.