[R] Treatment effects on measurements through time: how to tell when (in time) treatment has a significant effect?

[c_e_cressler]

Hi, 

The data (attached) I am looking at consists of measurements of growth rate
at different ages, for individuals in two treatments (control and infected).
What I want to know is whether and when (what age) the growth rate of
infected individuals is higher than the growth rate for control individuals.

The simplest way to approach this question is to just do a t-test at each
age, but because the growth rates at a given age depend on the growth rates
at previous ages before, that seems statistically invalid. I have looked at
some of the time series literature, but most of that seems more complicated
than what I am trying to do. What I would like to be able to say is
something like, "The growth rate of infected individuals is higher than
control individuals for ages 18-30."

Any help or insight would be greatly appreciated.

Thanks!
Clay

P.S. The data is provided as two matrices: the first column are the ages at
which data was collected, and subsequent columns are the growth rate
trajectories for different individuals.  cntl.grates.rda
<http://r.789695.n4.nabble.com/file/n4679911/cntl.grates.rda>  
inf.grates.rda <http://r.789695.n4.nabble.com/file/n4679911/inf.grates.rda>  



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