Aims: This article aims to report on the reliability and validity of an instrument aimed at measuring tutorial group effectiveness in PBL. Method: The items within the instrument are clustered around motivational and cognitive factors based on Slavin's theoretical framework. A confirmatory factor analysis (CFA) was carried out to estimate the validity of the instrument. Furthermore, generalizability studies were conducted and alpha coefficients were computed to determine the reliability and homogeneity of each factor. Results: The CFA indicated that a three-factor model comprising 19 items showed a good fit with the data. Alpha coefficients per factor were high. The findings of the generalizability studies indicated that at least 9-10 student responses are needed in order to obtain reliable data at the tutorial group level. Conclusion: The instrument validated in this study has the potential to provide faculty and students with diagnostic information and feedback about student behaviors that enhance and hinder tutorial group effectiveness.