In collaborative learning, evaluating the process involves teamwork dynamics, and assessing the product focuses on the accuracy and quality of the final output. Assessment plays a crucial role, as it defines and measures the effectiveness of group activities to ensure that learning objectives are met. Mediation analysis is an important technique to better understand relationships between variables, specifically designed to test hypotheses about potential causal effects in various areas. However, many research initiatives have been discontinued prematurely due to the Baron-Kenny data restrictions. This research takes a case study of online learning from the Portuguese Open University to determine if and how group selection and interaction frequency affect individual assessment. The contribution lies in applying quantitative causal mediation analysis to collaborative learning assessment. The Lambda Mediation Ratio is proposed to enhance mediation analysis by enabling quick and flexible categorization into full, partial, or no mediation. Using Moodle platform logs and student outcomes, it was possible to find a significant influence of group dynamics on academic performance, highlighting the practical application of this improved methodology in an educational context. These findings reassure us of the relevance and applicability of this research in real-world educational settings.