In association with

  • Bristol, North Somerset and South Gloucestershire Integrated Care Board
  • West of England Academic Health Science NetworkWest
  • National Institute for Health Research


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.

Observational study

A family of studies in which investigators compare people who take an intervention, with those who do not. The investigators neither allocate patients to receive the intervention nor administer the intervention. Instead, they compare records of patients who took an intervention and have been treated in routine practice, with similar patients who had not taken the intervention. The most common observational designs are case-studies, case-series, case-control studies, cohort studies, and historically-controlled studies. (Howick)


A ratio of events to non-events. If the event rate for a disease is 0.2 (20%), its non-event rate is 0.8 and therefore its odds are 2/8.

p value

The probability that a particular result would have happened by chance.


A placebo is a fake or inactive intervention, received by the participants allocated to the control group in a clinical trial. It is indistinguishable from the active intervention received by patients in the experimental group. One definition is that use of a placebo describes what happens when you do nothing, so that in the context of a clinical trial, for instance, a placebo group could describe the natural history of a disorder without the intervention under test. It is the effect we see when we do nothing.

Positive predictive value (+PV)

The proportion of people with a positive test who have disease.


This employs objectivity and is characterised by single truths; observation; measurable features and utilises quantitative methodologies. Its language is that of numbers.

Post-marketing surveillance

A procedure implemented after a drug has been licensed for public use, designed to provide information on the actual use of the drug for a given indication, and on the occurrence of side effects and adverse reactions. It’s a method for epidemiologic study of adverse drug reactions.

Post-test probability

The probability that a patient has the disorder of interest after the test result is known.

Pre-test probability

The probability that a patient has the disorder of interest prior to administering a test.


The baseline risk of a disorder in the population of interest.