The assessment of individual knowledge and abilities should be frequently undertaken when learning is developed in interactions with other students, such as in group work and/or cooperative learning. Previous research reveals that group work assessment is a neglected research area, and this applies in particular to group work assessment interventions studies. The focus of this article is methodological, and its aim is to provide a reflective and critical account of a group work assessment intervention project, and the implications of the different choices made in this process. The intervention project that was scrutinized had a mixed-method longitudinal quasi-experimental design, and interventions in the form of shorter educational sessions were central to the project. Both qualitative and quantitative data were collected, analyzed, and compiled. The methodological issues discussed and problematized were the importance of (a) establishing collaboration with teachers; (b) well-thought-out and delimited methodological choices, and subsequent consequences; and (c) including both teachers and students to secure successful effects of the interventions. As a result of the study, it was concluded that intervention could be beneficial as a means of increasing the scientific knowledge in relation to intervention studies, and also to the emerging discourse on group work assessment.