Considering the paramount importance of writing logical arguments for college students, this study investigated the effect of dynamic assessment (DA) of Toulmin Model through teacher- and collective-scaffolding on argument structure and overall quality of argumentative essays of Iranian EFL university learners. In so doing, 45 male and female Iranian EFL learners taking part in the study were randomly assigned into three groups (two experimental groups including the teacher- and collective-scaffolding and one control group), each consisting of 15 learners. Toulmin Model of argumentation was used as an instructional tool in this research. The necessary data were collected through a pre- and post-test argumentative essay. During the experiment, the dynamic assessment groups wrote and revised their essays in response to teacher’s or peers’ supportive dialogue and zone of proximal development (ZPD) sensitive feedback on the argument structure of their essays; whereas, the control group did not receive such mediation and they were evaluated on their own independent performance. The results of statistical analyses carried out on post-test scores on argument structure and overall quality of the essays pointed out to the outperformance of the teacher- and collective-scaffolding groups on both variables. Furthermore, follow-up Post-hoc analyses revealed no significant difference between the teacher- and collective-scaffolding groups in terms of the overall quality of the argumentative essays. However, the statistically significant difference between the two experimental groups with regard to the argument structure indicated the outperformance of the teacher-scaffolding over the collective group. The obtained results support the fact that autonomy and improvement cannot be thrust upon learners, rather they need to be assisted wisely towards independence.
