7. CONCLUSIONS
In this paper, we have made explicit the survey design and development process of the Newtonian Gravity Concept
Inventory (NGCI). Our methods followed the best practices outlined in Benson and Clark (1982) and guided the
organization of the paper. In Section 2, we outlined our approach to the Planning phase of instrument development,
reviewing the results of the qualitative foundation developed in Williamson and Willoughby (2012). In Section 3,
we described construction of four concept domains that included: (1) Directionality, (2) Force Law, (3)
Independence of Other Forces, and (4) Threshold. We also discussed the process by which multiple-choice items
were constructed. Section 4 described the test population and provided a Classical Test Theory statistical analysis
of the reliability of the NGCI as well as measures of item difficulty and discrimination. Section 5 highlights the
evolution of nine items to illustrate the iterative process by which we evaluated and modified the multiple-choice
questions to ensure the conceptual breadth, scientific accuracy, and item clarity of the NGCI. Section 6 uses
evidence from Williamson andWilloughby (2012) and Sections 3–5 to argue for the validity of the NGCI as a
robust instrument for measuring Astro 101 student understanding of Newtonian gravity.