Steps substantive researchers can take to build a scientifically strong case for the existence of trajectory groups.

Sterba and Bauer's Keynote Article does a superb job of reviewing the "... assumptions, strengths, and limitations of model-based person-oriented methods-clarifying which theoretical principles [researchers] can test and the compromises and trade-offs required to do so." Their writing is exceptionally clear, and the examples given highly instructive. At the same time, their arguments may be so convincing that the reader may be reluctant to pursue person-oriented analyses in a longitudinal context. The purpose of this Commentary is not to contradict Sterba and Bauer's arguments but to briefly review the steps that substantive researchers can take in building a scientifically strong case for either assuming continuously varied growth "... or that [trajectory groups] actually exist" according to Raudenbush. These steps have been elaborated in a series of papers by Muthén and colleagues, but it is useful to briefly review them here.