The first thing to realize is that there are different kinds of cancer studies. A clinical trial is a particular kind of cancer study, one that is both experimental and prospective.

What's an experimental study?

Experimental studies can be understood in contrast to observational studies.

In an experimental study, investigators ask participants to take something (such as a drug) or do something (such as attend a support group). Investigators then record what happens to the participants as a result. The "something" that participants take or do is called an intervention.

In an observational study, by contrast, there is no intervention. Investigators simply observe and record naturally occurring events: for example, the number of lung cancer cases that occur within a group of people who live or work in cities along the East Coast.

Observational studies are important, and can provide useful information about many issues such as risk factors for cancer. For example, the link between smoking and lung cancer was established through observational studies.

However, observational studies cannot be used to draw conclusions about how best to prevent or treat cancer. Prevention and treatment strategies need to be tested in experimental studies.

What's a prospective study?

Prospective studies can be understood in contrast to retrospective studies.

In a prospective study, investigators follow participants forward in time for weeks, months, or years and record what happens to them.

In a retrospective study, by contrast, investigators look back at what happened to a group of people in the past. For example, they may take information from participants' medical records or ask participants to recall what they ate or did during a defined period of time.

Retrospective studies, while helpful in cancer research, are of limited use in determining new medical care because information about what happened to participants in the past is often incomplete.

Prospective studies don't rely on the reconstruction of past events, so they are generally considered to produce more reliable results than retrospective studies.

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