[R] modelling a particular design

ripley@stats.ox.ac.uk ripley at stats.ox.ac.uk
Sat May 11 18:31:10 CEST 2002

A lot of basic information is missing.  For example, is y a continuous
measurement?  What do you mean by GLM? (Warning, SAS and R mean different
things.) ....

If y is indeed continuous, I think lme (in R's recommended package nlme)
will do what you want, with a random effect for subject.  It does need
some learning.  The Pinheiro & Bates book will help a lot, provided you
have the background knowledge they presume.

AFAIK there is no suitable software in Omegahat.

On Sat, 11 May 2002, Andreas Bartsch wrote:

> Dear R- and Omega-list-members,
> I am trying to make statistical inference about the following design:
> A dependent variable y has been measured multiple times, i.e. 4 times
> (y1,y2, y3, y4), unfortunately suffering from some successive dropouts (i.e.
> the sample sizes varies for y1, y2, y3, and y4). For every y, two other
> variables (covariates) were also measured: x & z, and both do presumably
> exert an effect on y. At some cutoffs, x can also be trichotomized into 3
> ordinal levels constituting a presumed factor (fx) influencing the level of
> y at all of the different measurings (1-4). x and fx are rather stable
> arcoss the 4 measurements whereas z is not. H0 is that x and fx are not
> influencing the level of y (i.e. explaining any variance of y) irrespective
> of  (i.e. controlled for) z at any of the measurements.
> (1) What would be a good way of testing for the hypothesis in the context of
> a GLM or a canonical regression analysis? I can see that in a parametric
> testing and for a single measurement of y a simple multiple linear
> regression (MLR) or an ANCOVA (or a RANCOVA in a nonparametric approach)
> would do the trick. However, I am not sure how to tackle the issue facing
> repeated and at least biologically somehow interdependent measurements and
> the goal to include as much measurements as possible even though the sample
> size differs. I thought about running an MLR for x and an ANCOVA for fx
> seperately for y1, y2, y3 and y4 but I am not sure if this would require a
> correction for multiple testings and if this is in fact the best approach at
> all. Pooling all 4 measurements, on the other hand, would mean to pretend
> that they were all derived from different subjects which is clearly not the
> case. So, basically I don´t really know how to treat the testing.
> (2) Is there an implementation in R or Omega to perform such testing? How
> would that run?
> Hopefully, the question isn´t too trivial to the list - I am not a
> statisticician and just fed up with the comercial stats software... Thank
> you very much in advance-
> Andreas Bartsch, MD
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Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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