Discussion: Conditional growth charts

1. Overview. Wei and He are to be congratulated on an innovative and important article. The conditional approach to growth charts described in their article is important in a practical sense, and the use of quantile regression is both natural and well motivated. We look forward to further application of their idea to actual practice, because the concept of “falling behind” in one’s growth cycle has two meanings: the usual standard growth chart, and the conditional growth chart described here. We have described the Wei and He approach to pediatricians, and they all grasped the essential clever idea immediately and were enthusiastic about the idea. Our commentary will focus on three aspects of the approach used by the authors, specifically (a) the use of unpenalized B-splines as described by the authors; (b) conditional versus marginal semiparametric modeling of longitudinal data; and (c) some alternative modeling approaches to “catchup” that may get at the issue more directly and flexibly.