Re: help with multivariate analysis

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Re: help with multivariate analysis

Spencer Graves
Dear Dr. Vidali:

          1.  I would first plot the data a number of different ways.  Maybe
you've already done plenty of this, but I have had a tendancy to rush to
the model I thought was the "best" without adequate data integrity
checks.  I typically want to start with normal probability (using
"qqnorm").  Outliers, the need for transformations, mixtures, etc., will
jump off the page when I do this.  Sometimes, I find that I didn't get
the data read properly.  Also, I may consider log(heart.frequency) and
1/heart.frequency as well as heart.frequency.  In most cases, I will
base the subsequent analysis on whichever looks most nearly normally

          2.  I will also want to make scatterplots of the two variables as
well as plotting each vs. time (with one line for each patient and
separate plots for the two groups, but on the same scale).  This can
tell me how much correlation to expect as well as what kinds of effects
I want to model.

          3.  Have you consulted Pinheiro and Bates (2000) Mixed-Effects Models
in S and S-Plus (Springer)?  The first half of that book describes how
to analyze data sets like you describe when effects are linear and
additive.  The latter half describes what to do with nonlinear effects.

          4.  If you still would like further assistance from this group,
please submit another post, but PLEASE do read the posting guide!
"".  Anecdotal evidence suggests
that posts more consistent with that guide tend to get more useful
replies quicker.

          hope this helps.
          spencer graves

Matteo Vidali wrote:

> dear R users,
> I need some help for multivariate analysis.
> I have 2  anaesthetic treatment groups (20 patients/group) where I
> register heart frequency and pressure for 60 min (repeated measures
> every 5 minutes). I would like to perform a test to check if treatments
> are different in controlling freq and pressures during the anaesthesia,
> but i would like to have also an overall measure and not only multiple p
> for different time intervals. I also think I should choose a test in
> which time is meaningful since the measures are not simple repeated
> measurements but measurements taken at specific time points.
> 1 million dollar question.... how to do in R?
> thanks in advance
> Dr Matteo Vidali
> Dep. of Medical Sciences
> University of East Piedmont "A. Avogadro"
> --
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> PLEASE do read the posting guide!

Spencer Graves, PhD
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