Table 1

Six methods for defining normal in biomedical science*.

method

Properties of the clinical data, sign, or test

Gaussian

The statistical distribution of the data is known to be normal in persons without the disease.

Percentile

Test result lies within a certain percentile of the possible range of results.

Diagnositic

Research has established the probability that the target disease is present, for a given range of test results.

Therapeutic

Research has shown that a specific treatment has a known probability of success for a given range of test results.

Risk factor

Research has shown that presence of a risk factor increases risk of a specified outcome (e.g. morbidity or mortality).

Culturally desirable

Carries strong socio-political expectations of normal appearance or behaviour.

Notes: *as described by Sackett et al (1997) and Smith (2002).

Abbott et al. BMC Musculoskeletal Disorders 2006 7:45   doi:10.1186/1471-2474-7-45