A variation of a randomised controlled trial in which a sequence of alternative treatment regimens is randomly allocated to a patient. The outcomes of regimens are compared, with the aim of deciding on the optimum regimen for the patient.
Negative predictive value (-PV)
The proportion of people with a negative test who are free of disease.
Nested case-control study
A case control study in which cases and controls are drawn from the population in a cohort study. As some data is already available about both cases and controls, the effects of some potential confounding variables are reduced or eliminated. In this type of case-control study, a set of controls is selected from subjects i.e. non-cases at risk at the time of occurrence of each case that arises in a cohort, thus allowing for the confounding effect of time in the analysis.
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Number needed to treat (NNT)
The number of patients who need to be treated to prevent one bad outcome. It is the inverse of the ARR: NNT=1/ARR. Numbers needed to harm (NNH) is the number of patients who, if they received the experimental treatment, would lead to one additional person being harmed compared with patients who receive the control treatment. It is calculated as 1/ARI.