Clinical trials are needed so that the safety and efficacy of different drugs and treatments can be evaluated. The trial must produce the most reliable results, often with little data, because for example, a particular disease is rare. There are various types of clinical trials, each suited for a different purpose. A fundamental distinction is between observational studies and more useful, focused and reliable randomized controlled trials. In observational studies, the investigators only observe associations (correlations) between the treatments experienced by participants and their health status or diseases.
Clinical trials to test new drugs are designed as randomized, double blind, and placebo-controlled.
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Randomized: Each study subject is randomly assigned to receive either the study treatment or a placebo.
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Blind: The subjects involved in the study do not know which study treatment they receive. If the study is double-blind, the researchers also do not know which treatment is being given to any given subject. This 'blinding' is to prevent biases, since if a physician knew which patient was getting the study treatment and which patient was getting the placebo, he/she might be tempted to give the (presumably helpful) study drug to a patient who could more easily benefit from it. In addition, a physician might give extra care to only the patients who receive the placebos to compensate for their ineffectiveness. A form of double-blind study called a "double-dummy" design allows additional insurance against bias or placebo effect. In this kind of study, all patients are given both placebo and active doses in alternating periods of time during the study.
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Placebo-controlled: The use of a placebo (fake treatment) allows the researchers to isolate the effect of the study treatment.
The aim of the trial is to obtain a statistically significant result showing a significant or not difference in outcome between the groups of patients who receive a treatment and those who receive a placebo or a different treatment.
The number of patients enrolled in a study has a large bearing on the ability of the study to reliably detect the size of the effect of the study intervention. This is described as the "power" of the trial. The larger the sample size or number of participants in the trial, the greater the statistical power.