The NICE glossary provides brief definitions and explanations of terms and jargon used on our site. The terms describe how we work and how our guidance is produced.

Our glossary excludes specific clinical and medical terms although we intend to include these in the longer term. If you cannot find the term you are looking for, please email us so that we can consider adding it to the glossary. New terms are added regularly.

The glossary is not used for indexing the site or for navigation - for information on this, please see the NICE taxonomy.

Some definitions and examples are based on those in the ‘HTAi consumer and patient glossary’, with thanks to Health Technology Assessment International.


Objective measure

A measurement that reduces the possibility of subjective interpretation by observers and study participants.


A research technique that involves watching, listening to and recording behaviours, actions, activities and interactions.

Observational study

Individuals or groups are observed or certain factors are measured. No attempt is made to affect the outcome. For example, an observational study of a disease or treatment would allow 'nature' or usual medical care to take its course. Changes or differences in one characteristic (for example, whether or not people received a specific treatment or intervention) are studied without intervening.

There is a greater risk of selection bias than in experimental studies.

Odds ratio

Odds are a way to represent how likely it is that something will happen (the probability). An odds ratio compares the probability of something in one group with the probability of the same thing in another.

An odds ratio of 1 between two groups would show that the probability of the event (for example a person developing a disease, or a treatment working) is the same for both. An odds ratio greater than 1 means the event is more likely in the first group. An odds ratio less than 1 means that the event is less likely in the first group.

Sometimes probability can be compared across more than two groups - in this case, one of the groups is chosen as the 'reference category', and the odds ratio is calculated for each group compared with the reference category. For example, to compare the risk of dying from lung cancer for non-smokers, occasional smokers and regular smokers, non-smokers could be used as the reference category. Odds ratios would be worked out for occasional smokers compared with non-smokers and for regular smokers compared with non-smokers. See also confidence interval, relative risk, risk ratio.

Off-label prescribing

A drug or device prescribed to treat a condition or disease for which it is does not have UK marketing authorisation. See also specific indication.

OPCS Classification of Interventions and Procedures Version 4.6

The Office of Population Censuses and Survey (OPCS)* Classification of Surgical Operations was first issued in 1987. It was devised to enable the gathering of data relating to main operative procedures during an episode of hospital care. There have been several updates since the first publication which have been informed by direct feedback from OPCS-4.6 users and the classification is no longer limited to surgical procedures but has been extended to include a wider range of interventions carried out on patients undergoing treatment. From 1st April 2011 the NHS mandated version is OPCS-4.6.

The NHS Classifications Service (NCS) of NHS Connecting for Health has responsibility for the support, maintenance, development and licensing of the OPCS Classification of Interventions and Procedures (OPCS-4.6) which is Crown copyright.

*No longer a current organisation.

Optimal practice review

NICE guidance to help the NHS identify tests or treatments that are not effective or not good value for money. Stopping using these treatments will save money that can be spent on providing care and treatments that are effective. Depending on the topic, optimal practice reviews can be part of the clinical guidelines or technology appraisals programmes. We are also publishing a series of recommendation reminders about past recommendations, to help NHS staff stop using tests and treatments that do not work well.


The impact that a test, treatment, policy, programme or other intervention has on a person, group or population. Outcomes from interventions to improve the public's health could include changes in knowledge and behaviour related to health, societal changes (for example, a reduction in crime rates) and a change in people's health and wellbeing or health status. In clinical terms, outcomes could include the number of patients who fully recover from an illness or the number of hospital admissions, and an improvement or deterioration in someone's health, functional ability, symptoms or situation. Researchers should decide what outcomes to measure before a study begins.


A document that summarises the findings from the evidence gathered for a multiple technology appraisal or interventional procedures guidance. It is used to inform the Appraisal Committee or Interventional Procedures Advisory Committee about the drug or treatment, or procedure so that the Committee can then write the draft guidance.

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This page was last updated: 15 April 2011

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Accessibility | Cymraeg | Freedom of information | Vision Impaired | Contact Us | Glossary | Data protection | Copyright | Disclaimer | Terms and conditions

Copyright 2014 National Institute for Health and Care Excellence. All rights reserved.