[R] Residuals, smoothers and estimates of noise

[Crabb, David]

We have a clinical measurement on patients over time. Each patient has
about 5 of these measurements over a period of two years, but the
measurement are not necessarily taken at equal space in time. We want to
use this data to establish test-retest variability. My first thought was
to look at the residuals from fitting simple linear regression (the
measurement may deteriorate over time and we wish to remove this trend
-assuming of course that it is linear). But then what residual do I
choose to represent the variability for each patient? Would it be good
to use the mean of these? Has anyone got any better solutions with some
known functions in R? Do running medians or other smoothers require the
data to be a time series with equal spaces between time points?

Any help/references/suggestions in R would be gratefully received.

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Dr. David Crabb
School of Science,
The Nottingham Trent University,
Clifton Campus, Nottingham. NG11 8NS
Tel: 0115 848 3275   Fax: 0115 848 6690