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Table 1 Six methods for defining normal in biomedical science*.

From: Lumbar segmental mobility disorders: comparison of two methods of defining abnormal displacement kinematics in a cohort of patients with non-specific mechanical low back pain


Properties of the clinical data, sign, or test


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


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


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


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.

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