Discussion
a reduced number
Our study aimed to compare the effects of 3D PDF (intervention) and 2D atlas (control) on teaching anatomic structures with different levels of complexity in video lectures to initial learners. Their effects on short-term and long-term retention were assessed.While there were non-significant differences with little effect sizes between the intervention group and the control group in the liver tests (neither in the immediate nor in the delayed), medium effect sizes showed superiority of 3D PDF over 2D atlas in the immediate and the delayed genitalia tests. The effect sizes, which were 0.76 in the immediate test and 0.63 in the delayed test, were close to be classified as a large effect (0.80) [Our results demonstrated that 3D PDF is superior to 2D atlas for teaching the anatomy of male genitalia, which is a more complex structure, yet not for teaching the liver anatomy, which is a less complex structure, both in terms of short-term retention and long-term retention. Therefore, it indicated that if the complexity of the anatomical structures is high, teaching with 3D PDFs provides a better environment for initial learners than teaching with 2D anatomy atlases.A previous research [3D PDFs offer considerable practical benefits over 2D atlases in anatomy education, even though they are relatively recent material in anatomy education, not widely used by either teachers or students. Our findings suggest that 3D PDFs may be a favorable choice for teaching complex anatomical concepts, as stereopsis has a critical role in learning anatomy [This study has some limitations. First, it is a single-center study in a country with its unique context, therefore it may not be generalizable. Second, due to the discontinuation of a number of participants and the resulting uneven distribution of students repeating Year-2 between the groups, it was necessary to analyze the long-term scores with a reduced number of participants. While our goal was to minimize participant dropout by maintaining communication through e-messaging groups during the study, dropouts might have led to find a non-significant 