In current online courses, most learning analytics techniques collect, analyze and display low-level data about the interactions of students with educational activities and resources. These data are used to detect students with difficulties in the course, as well as educational activities and resources which might be problematic. This paper presents the Precise Effectiveness Strategy (PES) which enables to calculate the effectiveness of students with educational activities and resources in online courses from low-level events in a quantitative way, taking into account different aspects of the learning context. The PES is particularized for specific educational activities and resources, which are video resources and parametric exercises. Finally, we propose some visualizations related to video and exercise effectiveness in a real course in physics.