This study aimed to investigate the application of Group Dynamic Assessment (GDA) to writing accuracy of EFL learners and explore whether secondary interactants could benefit from interactions between mediator and primary interactants. The idea of implementing DA (Dynamic Assessment) in dyads seems unworkable since teachers are required to teach the whole class (Guk & Kellog, 2007). Moreover, Lantolf and Poehner (2004) suggest a new approach to DA that is GDA, which involves applying DA with a large number of learners rather than individuals. Following a multiple case study design and interactionist DA, the development of ten students in a class of twenty five was tracked during the eight sessions of DA program. Data were collected though written artifacts, video-recording of interactions, interview, and observation. The results indicated that GDA was an effective way of helping learners overcome their linguistic problems and there were signs of microgenetic as well as macrogenetic development within the same DA session and across sessions. The present findings provide further insight into understanding how secondary interactants benefit from the interactions between mediator and primary interactants.
