A randomised controlled trial (RCT) is an experiment carried out under highly controlled conditions. It uses the same measurements over time, comparing two or more groups of people. Each group will undergo a different treatment or therapy, and the aim is to see whether these lead to changes. Researchers usually compare those having treatment with a control group who isn’t having treatment.

Whether people end up in one group or another is decided randomly. In some RCTs those receiving the therapy don’t know whether, or which therapy they’ve having. This is known as a single blind RCT. In some, the researchers don’t know which therapy, if any, is being given either. This is known as a double blind RCT. With comparisons of medicines this is more easy to do, as you can’t tell what is in a pill from its appearance. However, with a treatment like psychotherapy it’s not possible to keep the sort of therapy hidden. This is because psychotherapy is all about conscious interaction.

The RCT is seen to be the strongest form of scientific evidence to show cause-and-effect. It can show how effective a therapy is, and also how safe it is. Whether a condition gets worse or improves as a result of the therapy can be made clear through this type of study.

Many practical considerations need to be taken into account when designing, conducting, and analysing the results of an RCT. The numbers of people in each ‘arm’ may be quite high in order to be meaningful. As a result they tend to be quite costly.