Statistics Notes: Correlation, regression, and repeated data

In clinical research we are often able to take several measurements on the same patient. The correct analysis of such data is more complex than if each patient were measured once. This is because the variability of measurements made on different subjects is usually much greater than the variability between measurements on the same subject, and we must take both kinds of variability into account. For example, we may want to investigate the relation between two variables and take several pairs of readings from each of a group of subjects. Such data violate the assumption of independence inherent in many analyses, such as t …