  Traditional problem-based exams are not efficient instruments for assessing
the "structure" of physics students' conceptual knowledge or for providing
diagnostically detailed feedback to students and teachers. We present the Free
Term Entry task, a candidate assessment instrument for exploring the
connections between concepts in a student's understanding of a subject. In this
task, a student is given a general topic area and asked to respond with as many
terms from the topic area as possible in a given time; the "thinking time"
between each term-entry event is recorded along with the response terms. The
task was given to students from two different introductory physics classes.
Response term thinking times were found to correlate with the strength of the
association between two concepts. In addition, sets of thinking times from the
task show distinct, characteristic patterns which might prove valuable for
student assessment. We propose a quantitative dynamical model named the Matrix
Walk Model which is able to match many aspects of the observed data. One
particular feature of the data -- a distinct "spike" superimposed on the
otherwise log-normal distribution of most thinking time sets -- has not been
fit. The spike, other patterns observed in the data, and the proposed
phenomenological model could all benefit from a grounding in cognitive theory.
