Clinical trials can be designed as controlled studies or as observational studies.
They are different ways of investigating the effect of a treatment on patients.
In controlled trials, doctors give 'treatments' to the patients on the trial and measure the effect of those treatments.
In trials the 'treatment' can be a drug, a device, a therapy or even just a questionnaire.
In a randomised controlled study, the assignment of the 'treatment' to the patient is controlled using chance by flipping a coin or randomly generating a number on a computer.
Controlled trials always include two or more groups. In a simple trial, data from patients receiving the trial drug will be compared with data from patients in a control group who are receiving either the existing treatment or a placebo.
A placebo is a substance that has no medical effect on the patient. It is usually a capsule, pill or liquid made of starch, sugar or salt. It looks the same as the trial medicine.
Doctors use placebos to ensure that any differences in the results for the treatment group and the control group are due to the active substance in the new medicine.
In open-label trials, both the researchers and patients know which groups are receiving the trial and control treatments.
In single-blind, randomised controlled trials, patients do not know if they are receiving the trial or the control treatment.
In double-blind, randomised controlled trials, neither the doctor nor the patient know which patients are receiving the trial and the control treatments.
If feasible, double-blind, randomised controlled trials are the best way to test a new treatment. They reduce the risk that trial results will be affected by the doctors’ bias or actions. For example, doctors cannot put all the patients who are most likely to respond to the trial treatment into the group who will receive it.
In an observational study, the doctor just observes patients and takes measurements from them. The patients do not receive any treatments.
For example, the doctor may be interested in comparing the movements of patients who have been taking steroids since they were five years' old and those that have not. The doctor will only measure their movement and won't give them any new treatments.