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Re: st: Fractional Polynomials for Longitudinal Data

From   "Stas Kolenikov" <>
Subject   Re: st: Fractional Polynomials for Longitudinal Data
Date   Mon, 29 Dec 2008 08:57:48 -0600

On 12/28/08, <> wrote:
> I'm new to fractional polynomial models. Is there a longitudinal data option
> for mfracpol or fracpoly?

In what particular way do you think of introducing a longitudinal
aspect to this procedure? To me, -*fracpol*- models are mostly useful
for smooth interpolation leading to prediction, and frankly I won't
see terribly much use for them in causal micro level inference --
that's what you are probably most interested in with longitudinal
data. If you are interested in flexible formulations with longitudinal
data, I'd probably be looking towards say B-splines, and/or their
multivariate generalizations. They allow for the familiar (X'X)X'y
formulae (or whatever you can have for more complex models) in
(generalized) linear models (something you can put into -xtm*- models
as predictors), at the same time producing smooth curves through data.

Stas Kolenikov, also found at
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