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