  The Force Concept Inventory (FCI) has been widely used to assess student
understanding of introductory mechanics concepts by a variety of educators and
physics education researchers. One reason for this extensive use is that many
of the items on the FCI have strong distractor choices that correspond to
students' alternate conceptions in mechanics. Instruction is unlikely to be
effective if instructors do not know the common alternate conceptions of
introductory physics students and explicitly take into account students'
initial knowledge state in their instructional design. Here, we discuss
research involving the FCI to evaluate the pedagogical content knowledge of
both instructors and teaching assistants (TAs) of varying teaching experience.
For each item on the FCI, the instructors and TAs were asked to identify the
most common incorrect answer choice of introductory physics students. We also
discussed the responses individually with a few instructors. Then, we used the
FCI pre-test and post-test data from a large population (~900) of introductory
physics students to assess the pedagogical content knowledge of physics
instructors and TAs. We find that while both physics instructors and TAs, on
average, performed better than random guessing at identifying introductory
students' difficulties with FCI content, they did not identify many common
difficulties that introductory physics students have, even after traditional
instruction. Moreover, the ability to correctly identify students' difficulties
was not correlated with the teaching experience of the physics instructors.
