The students’ level of proficiency in any particular course is individually distinctive. Therefore, it is necessary for the educators to be able to address their student’s level of ability in understanding of the course they enrolled. Particularly, educators should be able to design a set of questions which suits the level of their students’ ability. For this reason, this study is concentrated on identifying the level of student’s ability in understanding probability concepts that has been included in the statistics course (STA150: Probability and Statistics 1). This course was enrolled by two groups of students (Group A and Group B) from the Faculty of Computer and Mathematical Sciences, Universiti Teknologi MARA Perak branch. Group A enrolled the course in December 2015 until March 2016 whilst Group B enrolled in June until November 2016 sessions. Since the aims of this study are to investigate the difference in students’ conceptual knowledge and understanding of probability concepts, as well as to examine which concepts were found most difficult by the students, hence the Rasch measurement approach was used to explore those aims. An instrument consists of 20 items in a test based on the “Counting Rule” topic was formed by an experienced lecturer to measure the level of student’s ability between the two groups. Based on the findings, it was found that there is a high reliability index of 0.93 (Group A) and 0.88 (Group B) which suggests the suitability of the instrument developed in this study to be replicated to the other samples, even though, the person student’s responses to the items for both groups appeared differently in terms of their difficulties in understanding the probability concepts which represents the student’s ability on this particular topic.