Background
The concept of fraction is one of the most difficult concepts of math in all levels of education. We aimed to investigate the effect of Merrill’s component display theory in the learning of students in math.
Materials and Methods: In this case-control study, the number of students under investigation was 183 boys where 31 students were randomly selected as experimental group and 30 as control group. The concept of teaching was designed based on the teaching design pattern of Merrill’s Component Display Theory and based on which the students in experimental group was taught for 10 sessions of 45 minutes; control group also received the traditional teaching. After two months, students were post-tested for their performance by a 10-question exam taken from teacher’s guidebook. Then, after three more weeks, they were again post-tested to ascertin their retention of the topic. Data were analyzed using SPSS software version 16.0
Results: The T-test results showed that the mean of students in experimental group increased about four grades and from 13.9 reached to the significant number of 18 for the performance post-test with the standard deviation of 0.63. In retention, the mean of students changed from 15.4 to 15 which is not a lot. It also means that there was no significant difference between mean scores of retention of Merrill’ group and the scores of post-test Merrill’s group. It means that education materials used Merrill’s design have been positively effective.    
Conclusion
Considering the findings, it can be stated that the Merrill’s component display theory has been effective in both performance and the retention of the students.