In this mixed methods study, the researchers developed a video-based measure called a "Prediction Assessment" to determine preservice elementary teachers' abilities to predict students' scientific reasoning. The instrument is based on teachers' need to develop pedagogical content knowledge for teaching science. Developing a knowledge base for aiding teachers in their abilities to predict students' scientific reasoning promotes student learning because it enables teachers to understand students' current conceptions and to be able to build lessons to improve upon those ideas. To determine whether preservice teachers are improving in their abilities to predict scientific reasoning it is necessary to have an instrument that can measure their current abilities and then to measure whether those abilities improve through instruction. In the second part of this study the authors used the prediction assessment to determine whether the traditional or new (Iterative Model Building, or IMB) field experience improved preservice elementary teachers' abilities to make reasonable predictions of student scientific reasoning. It was found that though preservice teachers in both the traditional and IMB field experience approaches improved their abilities to make predictions, there was a greater number in the IMB group who made reasonable predictions and based those predictions on student reasoning.