Despite growing interest in the studies on Zone of Proximal Development (ZPD), its operation in the forms of individualized and group-wide has been controversial. To cast some empirical light on the issue, this study was designed to quantitatively and comparatively study the applicability of the two scenarios of ZPD-based instructions to the writing complexity of Iranian EFL learners in terms of learners' types (low vs. high scorers). To this end, 118 EFL learners identified as homogeneous based on the TOEFL ITP test were randomly assigned into two equal experimental groups (individualized ZPD- and group-wide ZPD-based groups) which respectively received compatible instructions utilizing conference writing (as an individual scaffolding technique) and low- and high-structured writing templates (as a whole-class scaffolding technique). The treatments were allocated to the groups in a random manner within homogeneous blocks. Prior to and after the treatments, three tests (two writing tests and one ZPD test) were administered to measure both groups’ writing complexity and their ZPD levels. A ZPD test was also administered in the mid of treatments. The non-parametric analysis of covariance (ANCOVA) revealed that the individualized-ZPD-based instruction is constructive to the writing complexity of the low scorers, while the group-wide ZPD-based instruction shows not much constructive to the writing complexity of the high scorers. The findings pointed to the use of ZPD-based instructions in TEFL writing in meaningful contexts and thereby showing impacts made on the writing complexity of learners. The study, indeed, yields support to the feasibility of group-wide ZPD measurement along with an individualized ZPD approach.
