Each version of the NGCI showed an

Each version of the NGCI showed an improvement in Cronbach’s a reliability, with the final Version 3 post-test
value at a¼0.84. For comparison, post-test Cronbach’s a is 0.75 for the Lunar Phases Concept Inventory
(Lindell and Olsen 2002), 0.72 for the Star Properties Concept Inventory (Bailey 2006), and 0.78 for the Light
and Spectroscopy Concept Inventory (Schlingman et al. 2012). Figures 1 and 2 show CTT item difficulty and
item discrimination values, respectively, for questions that remain on Version 3 of the NGCI, including
accompanying Version 1 and Version 2 values where appropriate. One will note that most CTT item statistics for