# Modelling longitudinal data

We have longitudinal data on children(n<20) in which we measure different quantities A,B,C,D (like distance walked, time spent in school etc.). These are all continuous variables. We measure these variables daily and we have been measuring them for more than one and half years.

Now, based on these variables (A,B,C,D), I am trying to model individual growth trajectories of each child. Ideally, I want to have a single curve that explains how these variables have evolved with respect to time. For now, I am not concerned if different child have different trajectories.

I have been studying multi-level models to model growth curves. These techniques were not helpful particularly because I am not interested in modelling individual variables separately and my time series data is quite long(n>500 for each individual). I want to find a relation between all these variables and time.

I have been wandering for quite a time. I am looking for getting started papers or some ideas.

• I am not sure what you mean by "all these variables" other than "these variables separately". The way to find a relationship between some independent variables and a dependent variable is to model them one at a time. Sometimes you can do data reduction (e.g. with principal component analysis) but that doesn't seem right here (distance walked and time spent in school don't go together in any obvious way). I don't think there's a way to get a single curve that represents all this. Nov 28, 2012 at 0:38
• You also say both "I am trying to model individual growth trajectories" and "I am not concerned if different individuals have different trajectories" which is contradictory. Nov 28, 2012 at 0:39
• thanks @PeterFlom for your comment.. i am looking for a technique similar to pca that supports longitudinal analysis..as i have said i want to find a common trajectory that explains how these variables are evolving with time...i can fit these variables separately but are there ways i can "aggregate" these different fitted models to find a (sort of ) single model? .... for the second comment i mean growth trajectory of each child (see edits) Nov 28, 2012 at 3:52