Pre-test probability In diagnostic tests: The proportion of people with the target disorder in the population at risk at a specific time point or time interval. Prevalence may depend on how a disorder is diagnosed. Prevalence See Pre-test probability. Prior distribution In Bayesian statistics this is the probability distribution for a statistic based on previous evidence or belief. Primary care Healthcare delivered outside hospitals. It includes a range of services provided by GPs, nurses, health visitors, midwives and other healthcare professionals and allied health professionals such as dentists, pharmacists and opticians. Primary outcome The outcome of greatest importance, usually the one in a study that the power calculation is based on. Probabilistic analysis In economic evaluation, this is an analysis that uses a probability distribution for each input. In contrast, see Deterministic analysis. Product licence An authorisation from the MHRA to market a medicinal product. Prognosis A probable course or outcome of a disease. Prognostic factors are patient or disease characteristics that influence the course. Good prognosis is associated with low rate of undesirable outcomes; poor prognosis is associated with a high rate of undesirable outcomes. Prospective study A research study in which the health or other characteristic of participants is monitored (or followed up ) for a period of time, with events recorded as they happen. This contrasts with retrospective studies. Publication bias Publication bias occurs when researchers publish the results of studies showing that a treatment works well and don t publish those showing it did not have any effect. If this happens, analysis of the published results will not give an accurate idea of how well the treatment works. This type of bias can be assessed by a funnel plot. Quality of life See Health-related quality of life . 