6. PHASE FOUR: VALIDATION
While the reliability of the NGCI is measured with the Cronbach’s a statistic, the validity must be assessed via
an argument-based approach (Kane 1992). Reliability refers to the consistency of an instrument’s measurements,
and validity refers to the interpretation one assigns to that measurement. In this section, we outline assumptions
that we make about the interpretation of NGCI scores as a measurement of Astro 101 student understanding of
Newtonian gravity and discuss evidence to support those assumptions.