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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 pop-up definition.



Sample Size

The number of people included in a study. In research reports, sample size is usually expressed as "n." In general, studies with larger sample sizes have a broader range of participants. This increases the chance that the study's findings apply to the general population. Larger sample sizes also increase the chance that rare events (such as adverse effects of drugs) will be detected. detail


Using tests or other methods of diagnosis to find out whether or not a person has a specific disease or condition before it causes any symptoms. For many diseases (for example, cancers), starting treatment earlier leads to better results. The purpose of screening is to find the disease so that treatment can be started as early as possible. detail

Selection Bias

A type of bias caused by an error in the way people are assigned to groups in a clinical research study. This can occur when the study and control groups are chosen so that they differ from each other in ways that may affect the outcome of the study. detail


The ability of a test to identify correctly people with a condition. A test with high sensitivity will nearly always be positive for people who have the condition (the test has a low rate of false-negative results). Sensitivity is also known as the true-positive rate. detail

Side Effects

Any effects of a drug or treatment that are not wanted. Side effects may be temporary and go away when the drug is stopped. Sometimes they continue for a longer time, even when the drug is no longer being taken. detail


The ability of a test to identify correctly people without a condition. A test with high specificity will rarely be wrong about who does NOT have the condition (the test has a low rate of false-positive results). Specificity is also known as the true-negative rate. detail

Standard Treatment

The treatment or procedure that is most commonly used to treat a disease or condition. In clinical trials, new or experimental treatments sometimes are compared to standard treatments to measure whether the new treatment is better. detail

Statistical Analysis

The process of preparing the results or conclusion of a study. A statistical analysis usually is performed by doing mathematical calculations known as statistics. detail

Statistical Significance

A mathematical technique to measure whether the results of a study are likely to be true. Statistical significance is calculated as the probability that an effect observed in a research study is occurring because of chance. Statistical significance is usually expressed as a P-value. The smaller the P-value, the less likely it is that the results are due to chance (and more likely that the results are true). Researchers generally believe the results are probably true if the statistical significance is a P-value less than 0.05 (p<.05). detail


A research process in which information is recorded for a group of people. The information is known as data. The data are used to answer questions about a health care problem. detail

Study Population

The group of people participating in a clinical research study. The study population often includes people with a particular problem or disease. It may also include people who have no known diseases. detail


The length of time people live. Survival can show whether treatments for a disease can prevent or delay death. Survival is usually expressed in terms of how long the people in a group remain alive. It sometimes is summarized by calculating an average. Survival also can be summarized by the number of people who are still alive after a certain length of time, such as 5 years later. detail

Systematic Review

A summary of the clinical literature. A systematic review is a critical assessment and evaluation of all research studies that address a particular clinical issue. The researchers use an organized method of locating, assembling, and evaluating a body of literature on a particular topic using a set of specific criteria. A systematic review typically includes a description of the findings of the collection of research studies. The systematic review may also include a quantitative pooling of data, called a meta-analysis. detail