Large-scale standardized ESL tests such as the International English Language Testing System (IELTS) are widely used around the world to measure language proficiency of test takers and make different decisions based on their scores. Reading comprehension is an integral part of such tests which requires test takers to read passages and answer a set of questions. Although IELTS is a popular standardized test and is used for making critical decisions about test takers, very few attempts have been made to explore the validity of the exam and especially the reading part of the General Training Module. With this in mind, the purpose of the present study was to use a non-parametric item response theory model, called Mokken Scale Analysis (MSA), to examine the validity of the reading part of the General Training module of IELTS. To this end, item responses of 352 test takers to the reading comprehension test were analyzed. The results of item scalability, total scalability, and item-pair scalability showed that the reading part is a weak unidimensional scale. Using Monotone Homogeneity Model (MHM), monotonicity results also indicated that there are some items which violate the monotonicity assumption, although their values are insignificant. The analysis of unidimensionality using the AISP revealed that there are two scales and four unscalable items in the reading part. Therefore, Mokken scale analysis did not support the unidimensional structure of the reading part of the General Training module of IELTS.
