  Although an important goal of introductory physics labs is to train students
in scientific reasoning and critical thinking, currently there are no standard
tests in physics designed to assess such skills. We are in the process of
developing and validating the Physics Lab Inventory of Critical thinking
(PLIC), an assessment to probe students' critical thinking abilities in physics
lab courses. The instrument asks students to critique a set of experimental
methods and data and use them to evaluate a particular physical model (the
period of a mass on a spring). Currently, we are validating the closed-response
survey through interviews with students and present the results of 12 such
interviews here. We describe a trend that has emerged from these interviews,
with students' reasoning falling into three main patterns of behavior:
selecting all options, cuing on keywords, and critically analyzing. We have
found ways to shift students to the last and more desirable behavior. We
discuss ways in which these findings are likely relevant to the design of other
concept inventories.
