The placebo effect occurs when the study outcome is subjective and modifiable by psychological factors. For example, asthma patients are able to better tolerate treatment when given a placebo. Patients who have suffered from the condition may be more likely to respond positively to the treatment, thereby improving the patients' satisfaction with their condition.

A placebo is not a substitute for the actual drug, but it may be an important part of a trial to help measure a patient's response. A placebo may help the patient experience an improved mood and decrease the amount of stress-related chemicals in the body. It may also cause a person to experience a reduction in pain. This effect may occur through changes in diet, exercise, and rest.

Some researchers have found that the placebo may change a person's perception of pain. For example, the placebo may cause a person's brain to interpret a sharp pain differently. Another benefit of a placebo is that it can reduce risks associated with delayed treatment. Also, a placebo helps establish short-term effects.

In multicenter trials, it is important to keep the study subjects informed that the study may include a placebo. In addition, subjects must be aware of the medical alternatives available to them. The number of patients in a Clinical Trials determines the length of the trial. A larger number of patients will lead to a faster onset of benefits.

However, different patient's progress at different rates, so natural differences between groups may mask any differences in treatment rates. If thec number of patients in a trial is large enough, these natural differences will be canceled out. As a result, small differences in treatment rates will be more apparent.