  While introductory electricity and magnetism (E&M) has been investigated for
decades, research at the upper-division is relatively new. The University of
Colorado has developed the Colorado Upper-Division Electrostatics (CUE)
Diagnostic to test students' understanding of the content of the first semester
of an upper-division E&M course. While the questions on the CUE cover many
learning goals in an appropriate manner, we believe the rubric for the CUE is
particularly aligned to the topics and methods of teaching at the University of
Colorado. We suggest that changes to the rubric would allow for better
assessment of a wider range of teaching schemes. As an example, we highlight
one problem from the CUE involving the superposition principle. Using data from
both Oregon State University and the University of Colorado, we discuss the
limitations of the current rubric, compare results using a different analysis
scheme, and discuss the implications for assessing students' understanding.
