Classical Test Theory (CTT) is one of the most straightforward and often used statistical methods for evaluating
multiple-choice instruments (Ding and Beichner 2009). While CTT is highly sample dependent (Hambleton and
Jones 1993), it can offer valuable information about the reliability and item functioning of an instrument, and it
was used with each version of the NGCI to inform the development. Here, we will briefly review the basics of
CTT, and we refer readers interested in the theory to Crocker and Algina (1986) and to examples of how it has
been applied to instrument development in the discipline of Astronomy Education Research (Bailey 2006;
Wallace and Bailey 2010; and Schlingman et al. 2012).