Calculating correlation coefficients with repeated observations: Part 2—correlation between subjects

In earlier Statistics Notes1 2 we commented on the analysis of paired data where there is more than one observation per subject. It can 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.1 The appropriate analysis depends on the question we wish to answer. If we want to know whether an increase in one variable within the individual is associated with an increase in the other we can calculate the correlation coefficient within subjects.2 If we want to know whether subjects with high values of one variable also tend to have …