Multi-level repeated measures growth modelling using extended spline functions

This paper explores the fitting of multi-level models to growth data over a wide age range using a new class of extended spline models. These extend c onventional splines based on a '+' function representation by allowing vari able order functions and by including fractional polynomial terms. The work focuses on modelling human growth in height and head circumference with ex ample data sets. The procedures can be used with covariates and for compari ng population parameters. (C) 1998 John Wiley & Sons, Ltd.