Assessment of Structural Comparability and Analysis of Differential Item Functioning (DIF) and Differential Test Functioning (DTF) of TIMSS 2007 8th-Grade Science Test among Iranian and American Students

It is necessary for international tests, such as TIMSS and PIRLS, to have Structural Equivalence, also known as Structural Comparability. In other words, test items should be functionally identical for all the participant countries and groups. The present research was an attempt to examine the structural comparability of TIMSS 2007 8th-Grade Science Test and differential functioning of its items among Iranian and American students, as well as the effect of items with differential functioning on the performance of Iranian students. A combination of confirmatory factor analysis and Item Response Theory was used to analyze the data and answer research questions. The results of factor analysis indicated that the science test had structural comparability between the two groups. This finding suggests that both Iranian and American students use an identical conceptual framework to answer the test items. However, the results of differential item functioning analysis indicated that 62% of TIMSS 2007 8th-Grade Science Test items had differential functioning against Iranian students. In short, the poor performance of Iranian students in TIMSS 2007 Science Test cannot be attributed to differential item functioning, and the causes should be sought elsewhere. Ministry of Education officials and administrators should try to teach the key concepts of any field in an interrelated manner and to cultivate divergent and multidimensional thinking skills and ability in students by preparing appropriate educational contents. Moreover, attempts must be made to distance the system from the traditional method of teaching, which mainly consisted of theoretical lectures, and involve students in practical and laboratory activities.