In association with

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



Randomized controlled trial (RCT)

An epidemiological experiment in which subjects in a population are randomly allocated into groups, usually called study and control groups, to receive or not receive an experimental preventive or therapeutic procedure, manoeuvre or intervention. The results are assessed by rigorous comparison of rates of disease, death, recovery or other appropriate outcomes in the study and control groups.

Relative risk (RR) (or risk ratio)

The ratio of the risk of an event in the experimental group compared to that of the control group (RR=EER / CER). Not to be confused with relative risk reduction (see below).

Relative risk reduction (RRR)

The percentage reduction in events in the treated group event rate (EER) compared to the control group event rate (CER): RRR = (CER-EER) / CER.


The extent to which the method used to collect data about a variable, produces consistent results from the same subjects, reproducible at different times when there is no evidence of change in the subjects


The systematic collection, analysis and interpretation of data to answer a particular question in order to generate new knowledge by establishing a truth.

Research evidence

Studies can be categorised according to the questions they seek to answer, and the evidence for evidence-based commissioning could include the following study types:
Descriptive: to identify the qualities and distributions of variables
Taxonomic: to compare and classify variables into related groups or categories.

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