Item 6 on the NGCI, shown in Table 10, succinctly and efficiently probes the concept of floating as related to the
concept of gravity in relation to an atmospheric threshold without relying on students to explain the effect in
terms of orbital mechanics, which is not in the NGCI’s intended scope. Early versions of Item 6 were overly
difficult because they relied on non-gravity terminology. Specifically, interviews suggested that the connection
between “apparent weightlessness” and “moving at the same rate as the spacecraft” in Version 2 was not readily
accessible to students, as there are plenty examples of people on Earth moving at the same rate as a car or plane
who do not experience weightlessness. Additionally, it was found that the word “falling” that was used in
Version 1 generally means falling down for students, resulting in confusion and guessing. Not only is the final
version more straightforward in terms of students’ natural language, it more directly probes the idea of an
atmospheric threshold related to gravity specifically. Again, the CTT statistics in Figures 1 and 2 show that these
changes were effective in creating an item of appropriate difficulty and discrimination.