Glossary
Control group.
Direct answer
A control group does not receive the treatment under test but instead nothing, a dummy treatment or the usual care. It provides the yardstick for how the condition would have developed without the new intervention. Without that comparison, improvement cannot be separated from natural course and from the effects of attention.
Many complaints fluctuate on their own and improve after a low point. Treating exactly at the low point makes any measure look effective. This regression towards the mean is what makes a control group indispensable: it passes through the same time, the same observation and the same expectation, minus the active component.
In horses an untreated control group is harder to justify than in people, because the owner pays for taking part and will not leave a valuable animal without care. Comparisons are therefore often made against standard practice, or crossover designs are used in which every animal passes through both conditions in turn.
Randomised controlled trialPlaceboBlinding (single and double)
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