page_title: Reference range
section_title: Bimodal distribution
hierarchical_section_title: Reference range / Standard definition / Establishment methods / Bimodal distribution
context_page_description: In health-related fields, a reference range or reference interval is the range of values that is deemed normal for a physiologic measurement in healthy persons. It is a basis for comparison for a physician or other health professional to interpret a set of test results for a particular patient. Some important reference ranges in medicine are reference ranges for blood tests and reference ranges for urine tests.
  The standard definition of a reference range originates in what is most prevalent in a reference group taken from the general population. This is the general reference range. However, there are also optimal health ranges and ranges for particular conditions or statuses.
  Values within the reference range are those within the normal distribution and are thus often described as within normal limits. The limits of the normal distribution are called the upper reference limit or upper limit of normal and the lower reference limit or lower limit of normal. In health care–related publishing, style sheets sometimes prefer the word reference over the word normal to prevent the nontechnical senses of normal from being conflated with the statistical sense.
context_section_description: In case of a bimodal distribution (seen at right), it is useful to find out why this is the case. Two reference ranges can be established for the two different groups of people, making it possible to assume a normal distribution for each group. This bimodal pattern is commonly seen in tests that differ between men and women, such as prostate specific antigen.
caption: Bimodal distribution
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