THE SUBJECTS

The subjects are the basic experimental material. They will vary, in their average response to a treatment, and in their profile over time. An important part of the design of the experiment is to arrange the subjects so that major sources of variability do not, as far as possible, interfere with the treatment comparisons. This is most often accomplished by blocking , in which subjects which are similar to each other are arranged in groups. Treatment comparisons are then made within groups.

It can also be useful to measure covariates . These are variables which explain some of the variability between subjects in the variable(s) of interest in the experiment. This variability can then be accounted for in the analysis so that it doesn't interfere with treatment comparisons. Covariates are most often measured before any treatments are applied to the subjects.

An important choice in the experiment is how many subjects there should be. It should not be thought that lots of measurements on each subject is equivalent to having lots of subjects. Between subject variability is what matters when comparing treatments applied to different subjects. It can be a useful discipline to do some power calculations when choosing the number of subjects. This will involve calculating the probability of detecting a treatment effect of a specified magnitude in an experiment with a given number of subjects.

Antedependence Modelling 27.2.96