Statistics notes: Calculating correlation coefficients with repeated observations: Part 1--correlation within subjects

In an earlier Statistics Note1 we commented on the analysis of paired data where there is more than one observation per subject, as shown in table I. We pointed out that it could be highly misleading to analyse such data by combining repeated observations from several subjects and then calculating the correlation coefficient as if the data were a simple sample. This note is a response to several letters about the appropriate analysis for such data. View this table: TABLE I Repeated measurements of intramural pH and PaCO2 for eight subjects2 The choice of analysis for the data in table I depends on the question we want to …