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