by Algy Moncrieff

Created on: September 22, 2011   Last Updated: April 03, 2012

Dividing people (or other test subjects) into experimental and control groups is one of the most effective methods of proving causality, and the search for causality is the key issue in much of statistics.  The thing is, as statisticians will tell you over and over again, correlation does not imply causality.  In other words, if everyone who took a drug became less healthy than everyone who did not take it, this produces correlation but it cannot be proved that it was the drug that caused the deterioration.  For example, what if the people who took the drug were less healthy anyway and would have deteriorated whether they took the drug or not?

There are a few tricks that statisticians have of getting around this problem, and one such method is the use of randomised control trials.  The idea behind this is that researchers want to eliminate 'selection bias', that is, people deciding whether to have treatment or not based on choice, not randomness.  However, if the drug was not only given to the very ill, but to a random selection of healthy and unhealthy people, then if there is a change in their health this can be ascribed to the drug.  Allow me to explain.

A randomised control trial works as follows.  A large number of people are randomly allocated to one of two groups: the experimental (or treatment) group and the control group.  The groups can be of any size and will be unlikely to be the same size, as everyone should be allocated according to probability, rather than putting half in one group and half in another.  The experimental group is given the treatment, the control group is given a placebo.  Because of this, nobody knows which group they are in, which further eliminates any psychological effects that taking a drug or placebo might have.

Because the element of choice has been eliminated, any observed effect is now free from selection bias, and therefore researchers can infer causality if they are certain that the trial was indeed random.  This is because there is no reason for their selection of people except random probability.  The only thing these people have in common is the taking of the drug.  Thus, any change in their health must be down to the drug.  This evidence should be supported by the lack of a similar response in the control group.  If the control group responds as well, then something has gone wrong.

Researchers must be certain

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