We chose a ten-item free-response format as the best method to probe introductory astronomy student understanding
of gravity. This serves three purposes: (1) to explore different representations of gravity in a variety of
situations, (2) to allow for analysis via a constant comparative method, as discussed below, and (3) to gain information
for distracter responses for a multiple-choice test that represent typical student answers in their typical
language. In keeping with our goal of focusing on the causes, effects, and extent of gravity, the questions had little
to no emphasis on equations. In order to reliably measure a student’s mental model, we avoided questions
where students needed to have heard the answer before, instead emphasizing questions whose answers can be
deduced easily by a true understanding of gravity. These are generally different types of questions than those that
are asked in textbooks at the end of the chapter. The questions explore the strength of gravity in a variety of environments
and contexts—on the moon, in space, in freefall, on planets with orbiting bodies, etc.