Mehdi’s research interests include second language writing (its learning, teaching, and assessing), test validation (especially argument-based approach), research methods (especially mixed methods research), and issues related to (language) teaching and learning. He is the author of The Routledge Encyclopedia of Research Methods (Routledge, 2016) and Mixed Methods Research in Language Teaching and Learning (Equinox, 2017). His most recent co-edited volume titled Studies and Essays on the Learning, Teaching and Assessing Second Language Writing was published in 2020 by Cambridge Scholar Publishing.

Mehdi has supervised 22 doctoral students to completion. The doctoral graduates are now currently teaching in various universities across the world. He has also supervised more than 50 master’s thesis.