Glossary of terms

Confounding factor (confounder)

A confounder can distort the true relationship between two (or more) characteristics. When it is not taken into account, false conclusions can be drawn about associations. An example is to conclude that if people who carry a lighter are more likely to develop lung cancer, it is because carrying a lighter causes lung cancer. In fact, smoking is a confounder here. People who carry a lighter are more likely to be smokers and smokers are more likely to develop lung cancer.

Control group

A control group (of cells, individuals or centres, for example) serves as a basis of comparison in a study. In this group, no experimental stimulus is received.

Cross sectional study

This is an epidemiological study that describes characteristics of a population. It is ‘cross sectional’ because data is collected at one point in time and the relationships between characteristics are considered. Importantly, because this study doesn’t look at time trends, it can’t establish what causes what.

Diagnostic study

A diagnostic study tests a new diagnostic method to see if it is as good as the ‘gold standard’ method of diagnosing a disease. The diagnostic method may be used when people are suspected of having a disease because of signs and symptoms, or to try and detect a disease before any symptoms have developed (a screening method).

Ecological studies

In ecological studies, the unit of observation is the population or community. Common types of ecological study are geographical comparisons, time trend analysis or studies of migration.


Epidemiology is the study of factors that affect the health and illness of populations.


An experiment is any study in which the conditions are under the direct control of the researcher. This usually involves giving a group of people an intervention that would not have occurred naturally. Experiments are often used to test the effects of a treatment in people and usually involve comparison with a group who do not get the treatment.

Genome-wide association study

This study looks across the entire genetic sequence (genome) to identify variations in this sequence that are more common in people with a particular characteristic or condition and that may be involved in producing that characteristic or condition.

Levels of evidence

This is a hierarchical categorisation (ranking) of different types of clinical evidence. It is partly based on the type of study involved and ranks evidence according to its ability to avoid various biases in medical research. Several ranking schemes exist that are specific to the question posed in the research. Studies with the highest ranking are those that provide the best evidence that a result is true.

Examples of studies ranked in order from high-level to low-level evidence are:

  • systematic reviews
  • single randomised controlled trials
  • controlled trials without randomisation
  • prospective cohort studies
  • case-control studies
  • cross sectional studies
  • case series
  • single case reports

The expert opinions of respected authorities, based on clinical experience, descriptive studies, physiology, bench research or first principles are often thought of as the lowest level evidence. Although there are different systems, some of which take into account other aspects of quality including the directness of the research, the levels are designed to guide users of clinical research information as to which studies are likely to be the most valid.

Likert scale

A Likert scale is a commonly used rating scale that measures attitudes or feelings on a continuous linear scale, usually from a minimum ‘strongly disagree’ response to a maximum ‘strongly disagree’ response, or similar. Likert-scales can be 5-point, 6-point, 10-point, etc. depending on the number of response options available.

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