The aim of this study is to determine whether items from the mathematics section of the 2012 Level Determination Exam indicate item bias according to gender and school type. In particular, the process of item bias has been determined using the Delphi technique and focus group interviews. A two-stage mixed method research has been used for the study. While the first stage consists of identifying items that display differential item functioning (DIF) according to gender and school type, the second stage consists of determining the sources of DIF using the Delphi technique and examining through a focus-group interview which DIF sources lead to item bias. Mantel-Haenszel and logistic regression methods have been used for DIF analysis. While two items with significant DIF were detected according to gender, five items in favor of private schools were detected according to school type. In the process of item bias, the reasons why items display DIF have been determined using the Delphi technique, and 22 DIF sources were agreed upon. Finally, an expert panel was made to examine whether the DIF sources are grounds for item bias or not. According to the panel of experts, one item according to gender and two items according to the school type have been determined to show bias.