Most doctors rely on their education and experience: what they have learned from their training, from their colleagues, and from diagnosing and treating people with similar problems. Doctors also read medical books and journals, consult with colleagues, and refer to other resources, such as authoritative health sites on the Internet, to get more information about specific problems and to keep up with new information generated by medical research. They also review recommendations (practice guidelines) published by groups of experts.

People who need health information rely on their doctors. But many people also turn to an ever-growing number of resources in print and on the Internet (see also Making the Most of Health Care: Researching a Disorder).

Research Studies: As new research findings are published, doctors evaluate the studies and consider how their findings might best be applied. Different types of studies provide different types of information.

• A cross-sectional study compares groups of people at the same point in time. Such a study may compare test results of people who do and do not have a given disease and is often used to evaluate how well tests help diagnose diseases.
• A case-control study compares the histories of people who do and do not have the condition that is being studied but otherwise seem to be similar. Such a study is often used to understand the cause or causes of an uncommon disease.
• In a cohort study, people are studied for a similar period of time, which varies from hours to decades, depending on what is being studied. A cohort study observes people with something in common (usually a disease) over time. This type of study may be used to determine the effect of a disease on people over time (prognosis).

A clinical trial is considered the most accurate type of study. In a controlled clinical trial, people entering the study are divided randomly (by chance) into two or more groups. One group of people receives a particular treatment or test, while the other groups (called control groups) receive different treatments or tests or no treatment or test at all. The random assignment ensures that the different groups are as similar as possible. That way, any difference in outcome is likely to be due to the treatment or test being studied rather than an underlying and potentially unknown difference between the groups (see The Science of Medicine and Clinical Trials: Introduction).

Did You Know... Many of the resources doctors use are available to consumers. A clinical trial in which people are randomly assigned to groups for evaluation is considered the most accurate type of research study.

Sometimes studies are done to compare the relative costs of different approaches to diagnosis and treatment. These studies are called cost-effectiveness or cost-benefit studies. They may help doctors consider the effects of decisions from society's perspective, but they may be less helpful when doctors are considering decisions for a particular person.

Because of differences in the way studies are planned and carried out, even studies that are intended to evaluate the same thing may produce conflicting results. One method to try to resolve these conflicts is to prepare a summary of the findings of all studies that pertain to the topic and rigorously compare and evaluate them. This type of study is called a systematic review. Another method that tries to resolve conflicting results of studies is called meta-analysis. A meta-analysis mathematically combines the results of many studies.

Last full review/revision June 2007 by Thomas V. Jones, MD, MPH