The addition of these three questions to Version 3 represents an effort to probe a wider range of student ideas in
the Directionality Domain. Student ability to reason about direction in Versions 1 and 2 of the NGCI was mainly
explored with questions about non-spherical objects. While these types of questions proved qualitatively
valuable in the analysis described in Williamson and Willoughby (2012), introducing questions that use relative
motion and superposition expanded the scope of the NGCI in measuring student understanding of how direction
of gravitational force is determined.