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 Glossary of Terms
Glossary of Terms
We know that many of the concepts used on this site can be difficult to understand. For that reason, we have provided you with a glossary to help you make sense of the terms used in Comparative Effectiveness Research. Every word that is defined in this glossary should appear highlighted throughout the Web site. When you come upon a highlighted term and would like to read the full definition, you can either click on the word to visit the glossary or roll your mouse over the word for a popup definition.
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 Placebo

An inactive substance commonly called a "sugar pill." In a clinical trial, a placebo is designed to look like the drug being tested and is used as a control. It does not contain anything that could harm a person. detail
 Placebo Controlled Study

A study in which the effect of a drug is compared with the effect of a placebo (an inactive substance designed to resemble the drug). In placebo controlled clinical trials, participants receive either the drug being studied or a placebo. The results of the drug and placebo groups are then compared to see if the drug is more effective in treating the condition than the placebo is. detail
 Placebo Effect

A physical or emotional change that occurs after a participant in a research study takes a placebo. The change, which may include the lessening of symptoms, is not the result of chemical effects of the placebo because the placebo does not contain any active ingredients. The change is often based on the participant's or researcher's expectation that a change will occur. detail
 Pooled Odds Ratio

When the data on odds ratios from multiple studies are combined, the result is a pooled odds ratio (POR). An odds ratio (OR) is the comparison of the chance of an event occurring in one group to the chance of it occurring in another group. The odds ratio is a measure of effect size and is commonly used to compare results in clinical trials. detail
 Positive Predictive Value

Indicates the likelihood that a person with a positive test result would actually have the condition for which the test is used. The higher the value of the positive predictive value (for example, 90 percent would be considered a high value), the more useful the test is for predicting that the person has the condition. detail
 Pretest Probability

The probability that a person has a particular disease before any test results are obtained. The pretest probability for large groups of people (such as the population of a city) is the same as the prevalence of the disease in that group. detail
 Prevalence

How often or how frequently a disease or condition occurs in a group of people. Prevalence is calculated by dividing the number of people who have the disease or condition by the total number of people in the group. detail
 Prior Probability

The probability that a person has a particular disease before any test results are obtained. The prior probability for large groups of people (such as the population of a city) is the same as the prevalence of the disease in that group. detail
 Probability

The likelihood (or chance) that an event will occur. In a clinical research study, it is the number of times a condition or event occurs in a study group divided by the number of people being studied. detail
 Prognosis

The way a health situation is likely to turn out. Prognosis refers to the usual progression and outcome of a condition. detail
 Prospective Observational Study

A clinical research study in which people who presently have a certain condition or receive a particular treatment are followed over time and compared with another group of people who are not affected by the condition. detail
 Publication Bias

The tendency of researchers to publish experimental findings that have a positive result, while not publishing the findings when the results are negative or inconclusive. The effect of publication bias is that published studies may be misleading. When information that differs from that of the published study is not known, people are able to draw conclusions using only information from the published studies. detail