Empirical rule – The 68-95-99.7 Rule

Empirical rule

Empirical rule

The 68-95-99.7 Rule
For Normal Distributions

This rule applies generally to a variable X having normal (bell-shaped or mound-shaped) distribution with mean "mu" (the greek letter) and standard deviation "sigma" (the greek letter). However, this rule does not apply to distributions that are not normal.

  • Approximately 68% of the observations fall within 1 standard deviation of the mean.
  • Approximately 95% of the observations fall within 2 standard deviations of the mean.
  • Approximately 99.7% of the observations fall within 3 standard deviations of the mean.

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