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Statistics notes: The normal distribution

BMJ 1995; 310 doi: (Published 04 February 1995) Cite this as: BMJ 1995;310:298
  1. Douglas G Altman, Heada,
  2. J Martin Bland, reader in medical statisticsb
  1. a Medical Statistics Laboratory, Imperial Cancer Research Fund, PO Box 123, London WC2A 3PX
  2. b Department of Public Health Sciences, St George's Hospital Medical School, London SW17 0RE
  1. Correspondence to: Mr Altman.

    When we measure a quantity in a large number of individuals we call the pattern of values obtained a distribution. For example, figure 1 shows the distribution of serum albumin concentration in a sample of adults displayed as a histogram. This is an empirical distribution. There are also theoretical distributions, of which the best known is the normal distribution (sometimes called the Gaussian distribution), which is shown in figure 2. Although widely referred to in statistics, the normal distribution remains a mysterious concept to many. Here we try to explain what it is and why it is important.

    FIG 3

    (left)—Serum albumin values in 248 adults FIG 2 (right)—Normal distribution with the same mean and standard deviation as the serum albumin values

    In this context the name “normal” causes much confusion. In statistics it is just a name; …

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