Glossary

• Mean: The average value.
• Median: The middle value in a list of numbers in numerical order.
• Confidence Interval (CI): The range around a study’s result within which we would expect the true value to lie. CIs account for the sampling error between the study population and the wider population the study is supposed to represent.
• Number-Needed-to-Treat: The number of patients with particular condition who must receive a treatment in order to prevent the occurrence of one adverse outcome.
• Power: In a comparison of two interventions, the ability to detect a difference between the two experimental if one in fact exists.
• Type I Error: False positive; Occurs if an investigator rejects a null hypothesis that is actually true in the population.
• Kaplan Meier Analysis: A curve that starts at 100% of the study population and shows the percentage of the population still surviving (or free of disease or some other outcome) at successive times for as long as information is available.
• Hazard Ratio: The ratio of the hazards in the treatment and control groups, where the hazard is the probability of having the outcome at time t, given that the outcome has not occurred up to time t.
• Logistic Regression: A method for determining the relationship between predictor variables and a dichotomously coded dependent variable.
• Odds Ratio (OR): Ratio of the odds (those with the outcome divided by those without it) in the treatment group to the corresponding odds in the control group. An odds ratio of 1 implies that the outcome is equally likely in both groups.
• Relative Risk: Ratio of the rates of outcomes in the treatment and control groups. This expresses the risk of the outcome in the treatment group relative to that in the control group.
• Absolute Risk: Risk stated without any context.
• Meta-Analysis: A systematic review which uses quantitative methods to summarize the results.
• Selection Bias: Error due to systematic differences in characteristics between those who are selected for study and those who are not. It invalidates conclusions and generalizations that might otherwise be drawn from such studies.
• Publication Bias: A bias in a systematic review caused by incompleteness of the search, such as omitting non-English language sources, or unpublished trials (inconclusive trials are less likely to be published than conclusive ones, but are not necessarily less valid).
• Levels of Evidence: A hierarchy of study designs according to their internal validity, or degree to which they are not susceptible to bias.