“…Even within seemingly homogeneous groups of learners, different trajectories may emerge, based on differences between students regarding how they acquire knowledge. Wilson's (1989) Saltus model addresses these different growth rates in different populations (Draney & Wilson, 2007;Wilson & Draney, 1997), multidimensional IRT (MIRT) models address them in multiple groups (Xu & von Davier, 2006), and growth-curve models address them in an IRT mixture (Meiser, Hein-Eggers, Rompe, & Rudinger, 1995;Rijmen, de Boeck, & Maas, 2005).…”