Dynamic Assessment (DA) refers to a range of approaches that incorporate mediation into the assessment procedure (Poehner, 2008). Although DA has been applied to some areas of second language pedagogy, its effect on speaking skill seems as if to be less attended to, hence the present study aims at investigating the effects of this assessment procedure on the Iranian advanced EFL learners speaking skill proficiency. To this end, 40 homogenous advanced EFL learners were divided into three groups. They were assigned to two DA groups and one Non- DA group. As the pretest, the participants of the three groups were interviewed to assess their speaking proficiency. Next, the Non- DA group participants were given specific topics as discussion topics and were required to discuss them in the class without any DA based intervention. The first DA group’s participants were assessed and given the required assistance through interaction based DA procedures, while the second DA group received DA based intervention following Lantolf and Poehner (2011) scale to assess and assist the participants’ speaking proficiency in their discussions. The results of two Matched Samples t-test indicated that: (a) interactionist model of DA had statistically significant positive effect on Iranian EFL learners’ speaking ability; (b) interventionist model of DA had statistically significant positive effect on Iranian EFL learners’ speaking ability. Furthermore, the results of ANCOVA indicated that the three groups, namely, interactionist DA, interventionist DA, and non- DA had statistically significant different effects on Iranian EFL learners’ speaking ability with the interactionist DA group outperforming. The findings of this study can shed light on the comparative efficacy or inefficiency of the DA based assessment procedures in the realm of foreign language teaching and testing.
