Glossary of terms

Longitudinal study

A longitudinal study is one that studies a group of people over time.


This is a mathematical technique that combines the results of individual studies to arrive at one overall measure of the effect of a treatment.

Narrative review

A narrative review discusses and summarises the literature on a particular topic, without generating any pooled summary figures through meta-analysis. This type of review usually gives a comprehensive overview of a topic, rather than addressing a specific question such as how effective a treatment is for a particular condition. Narrative reviews do not often report on how the search for literature was carried out or how it was decided which studies were relevant to include. Therefore, they are not classified as systematic reviews.

Negative predictive value

This is one of a set of measures used to show the accuracy of a diagnostic test (see sensitivity, specificity and positive predictive value). The negative predictive value (NPV) of a test is a measure of how accurate a negative result on that test is at identifying that a person does not have a disease. The NPV is the proportion of people with a negative test result who do not truly have a disease. For example, if a test has an NPV of 75%, this means that 75% of the people who test negative are truly disease free, while 25% who test negative have the disease (false negatives). The NPV for a test varies depending on how common the disease is in the population being tested. An NPV is usually lower (false negatives are more common) when disease prevalence is higher.

Nested case-control study

A nested case-control study is a special type of case-control study in which ‘cases’ of a disease are drawn for the same cohort (population of people) as the controls to whom they are compared. These studies are sometimes called case-control studies nested in a cohort or case-cohort studies. The collection of data on the cases and controls is defined before the study begins.

Compared with a simple case-control study, the nested case-control study can reduce 'recall bias' (where a participant remembers a past event inaccurately) and temporal ambiguity (where it is unclear whether a hypothesised cause preceded an outcome). It can be less expensive and time consuming than a cohort study. Incidence and prevalence rates of a disease can sometimes be estimated from a nested case-control cohort study, whereas they cannot from a simple case-control study (as the total number of exposed people (the denominator) and the follow up time are not usually known).

Non-randomised study

In this type of study, participants are not randomly allocated to receiving (or not receiving) an intervention.

Observational study

In an observational study, researchers have no control over exposures and instead observe what happens to groups of people.

Odds ratio

An odds ratio is one of several ways to summarise the association between an exposure and an outcome, such as a disease. (Another commonly used approach is to calculate relative risks.)

Odds ratios compare the odds of the outcome in an exposed group with the odds of the same outcome in an unexposed group. Odds tell us how likely it is that an event will occur compared to the likelihood that the event will not happen. Odds of 1:3 that an event occurs, e.g. that a horse wins in a race, means the horse will win once and lose three times (over four races). Odds ratios are a way of comparing events across groups who are exposed and those who aren't.

Open label

Open label means that investigators and participants in a randomised controlled trial are aware of what treatment is being given and received (the study is not blinded).

Peer review

Peer review involves giving a scientific paper to one or more experts in that field of research to ask whether they think it is of good enough quality to be published in a scientific journal. Studies that are not of sufficient quality will not be published if their faults are not corrected. Journals that use peer review are considered to be of better quality than those which do not.

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