Abstract
Control groups are expected to show what happens in the absence of the intervention of interest (negative control) or the effect of an intervention expected to have an effect (positive control). Although they usually give results we can anticipate, they are an essential component of all experiments, both in vitro and in vivo, and fulfil a number of important roles in any experimental design. Perhaps most importantly they help you understand the influence of variables that you cannot fully eliminate from your experiment and thus include them in your analysis of treatment effects. Because of this it is essential that they are treated as any other experimental group in terms of subjects, randomisation, blinding, etc. It also means that in almost all cases, contemporaneous control groups are required. Historical and baseline control groups serve a slightly different role and cannot fully replace control groups run as an integral part of the experiment. When used correctly, a good control group not only validates your experiment; it provides the basis for evaluating the effect of your treatments.
Highlights
As Donald Rumsfeld famously said about weapons of mass destruction, there are known knowns, known unknowns and unknown unknowns
A good control group allows you to do this – a bad control group means you cannot make valid comparisons to evaluate the activity of your test condition and, even worse, means you may end up drawing invalid conclusions
Positive controls are in a comparable position to your test treatment: they need a good negative control to be of any use
Summary
Correct randomisation to avoid bias is a basic but essential part of any experimental design (see chapter “Blinding and Randomization”) that applies to control groups. Control groups should be treated like any other experimental group within an experiment. Subjects for the control groups must come from the same population as the other groups so that the experiment is carried out on a homogenous population. This means that normally they should not be historical values nor baseline values. As we will see below, there are specific circumstances where this is either not possible or where some flexibility is permitted
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