A clinical trial is a study carried out in human volunteers in order to answer specific health-related questions. There are two main types of clinical trials: interventional (also called experimental) and observational. Interventional trials examine the effects of a particular treatment in a controlled environment. A controlled environment is one where the investigators try to eliminate factors that could interfere with the treatment. For example, volunteers who are put into a controlled environment may be asked to eat the exact same food at the exact same time of day. In an interventional trial, people are assigned to groups: one group receives the treatment, and the other does not. Observational trials look at health issues in a "natural" environment (one that is not carefully controlled). Rather than assigning people to groups and determining who will receive a treatment, they observe people who have already chosen whether to take or not to take a treatment.
The main type of interventional trial is called the randomized controlled trial (RCT). RCTs are usually designed to compare two groups: an intervention group and a control group. The intervention group receives the treatment that is being studied, such as a new medication or a new way of using an existing medication. The control group gets a placebo (a "sugar pill" with no active ingredients) or a comparison treatment (usually whichever medication is the current "gold standard"). Study participants are randomly assigned to their groups, so that the results of the interventional trial will be directly related to the treatment that study participants are receiving, and less likely to be influenced by other factors.
There are a number of different types of observational trials, including cohort studies, case control studies, case series, and case reports. A cohort study starts with a large group of people who are taking a particular treatment, and follows them forward in time to see the results. Results are compared to another similar group that is not taking the treatment. A case control study starts with a group of people who have experienced a particular outcome (such as developing a disease) and looks backward in time to see whether they were exposed to the treatment being studied or not. A case report is a comprehensive summary of the treatment received by a particular patient. A case series is a group of case reports published together as a single paper.
Doctors and researchers generally consider data gathered from a RCT to be the most reliable, followed by data from cohort studies, case control studies, and case series. Different types of these clinical trials may be appropriate to answer certain research questions. For example, sometimes it is more useful to use a case control study design when studying a treatment side effect that is rare.
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