Homogeneity See heterogeneity. The rate at which an event occurs in a defined population over time. To be distinguished from prevalence . Patients assigned to a particular treatment group by the study protocol should be retained in that group for the purpose of analysis of the study results no matter what happens. Patients redefined or dropped from a study early on as a result of protocol violations unlikely to create bias may validly be considered exceptions to this rule.
Internal validity See validity .
An operator defined as the percentage of patients positive by gold standard for a particular disease, condition or injury who have a particular test result divided by the percentage of patients without the problem who have that same test result. A likelihood ratio of two means that the test result in question is twice as likely to come a patient with the problem as it is from a patient without the problem. The LR may be derived from reported sensitivity and specificity or from a clear understanding of the above definition. A review of a focused clinical question following rigorous methodological criteria and employing statistical techniques to combine data from independently performed studies on that question. To learn more, see the User's Guide.