Glossary of terms

Qualitative research

Qualitative research uses individual in-depth interviews, focus groups or questionnaires to collect, analyse and interpret data on what people do and say. It reports on the meanings, concepts, definitions, characteristics, metaphors, symbols and descriptions of things. It is more subjective than quantitative research and is often exploratory and open-ended. The interviews and focus groups involve relatively small numbers of people.

Quantitative research

Quantitative research uses statistical methods to count and measure outcomes from a study. The outcomes are usually objective and predetermined. A large number of participants are usually involved to ensure that the results are statistically significant.

Randomised controlled trial (RCT)

This is a study where people are randomly allocated to receive (or not receive) a particular intervention (this could be two different treatments or one treatment and a placebo). This is the best type of study design to determine whether a treatment is effective.

Randomised crossover trial

This is a study in which people receive all of the treatments and controls being tested in a random order. This means that people receive one treatment, the effect of which is measured, and then 'cross over' into the other treatment group, where the effect of the second treatment (or control) is measured.

Recall bias

Recall bias is when a person’s recall of their exposure to a suspected disease risk factor could be influenced by the knowledge that they are now suffering from that particular disease. For example someone who has suffered a heart attack may recall having a highly stressed job. The stress that they now report experiencing may now be subtly different than the stress they would have reported at the time, before they developed the disease.

Relative risk

Relative risk compares a risk in two different groups of people. All sorts of groups are compared to others in medical research to see if belonging to a particular group increases or decreases the risk of developing certain diseases. This measure of risk is often expressed as a percentage increase or decrease, for example ‘a 20% increase in risk’ of treatment A compared to treatment B. If the relative risk is 300%, it may also be expressed as ‘a three-fold increase’.

Retrospective study

A retrospective study relies on data on exposures and/or outcomes that have already been collected (through medical records or as part of another study). Data used in this way may not be as reliable as data collected prospectively as it relies on the accuracy of records made at the time and on people’s recall of events in the past, which can be inaccurate (referred to as recall bias).

Secondary analysis

A secondary analysis is when researchers revisit data that was collected for a different reason and analyse it again to answer a new research question. This type of analysis is sometimes prone to errors.

Selection bias

Selection bias is a distortion of evidence or data that arises from the way that the data is collected.


This is one of a set of measures used to show the accuracy of a diagnostic test (see specificity, negative predictive value and positive predictive value). Sensitivity is the proportion of people with a disease who are correctly identified as having that disease by the diagnostic test. For example, if a test has a sensitivity of 90%, this means that it correctly identified 90% of the people with the disease, but missed 10% (these people were ‘false negatives’ on the test).

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