This study investigates preservice middle school mathematics teachers' understanding of statistics and probability and provides a cognitive diagnostic assessment of their strengths and weaknesses on these subjects. A statistical reasoning test that included 15 multiple-choice and 5 open-ended items was developed from the perspective of the log-linear cognitive diagnosis model, which is a general form of the cognitive diagnosis models. The statistical reasoning test was applied to 456 preservice teachers from 4 universities in 3 different regions of Turkey. The collected data were analyzed using the Mplus 6.12 software, and diagnostic feedback on the preservice teachers' responses was provided based on the findings. The analysis suggested that although many preservice teachers were able to represent and interpret the given data, most experienced difficulty in drawing inferences about populations based on samples, selecting and using appropriate statistical methods, and understanding and applying the basic concepts of probability. In addition, preservice teachers had difficulty answering open-ended items. Implications for teaching are also discussed.