We report on the investigation of multiple sets of data from an electric circuits Physics by Inquiry course on students' ranking of topic difficulty. Students ranked the difficulty of the preceding class almost every class day and they ranked the difficulty of various course sections on a diagnostic (one diagnostic per section). In the OSU Physics by Inquiry (PbI) class, students--a majority of education undergraduates--work in groups, and are checkpointed as they do experiments in a section. In addition, there is a question of the day at the beginning of almost every class. Here, students are also asked to rank the difficulty, but of the preceding day's classwork. These "difficulty rankings" and student grades (used as a measure of performance) constitute our dataset. We compiled data from four sections of the Spring 2006 and one section of the Spring 2007 Physics by Inquiry electric circuits class. The sections on potential difference appear to be the most difficult.