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Fundamentals of Statistics 1: Basic Concepts :: The Empirical Rule
The standard deviation is the way variability is often described. If the data we are working with are roughly bell-shaped and symmetrical (looking like a normal distribution) then we can use the standard deviation to tell approximately how much of the data will cluster around the mean.

The so called empirical rule states that the bulk of the data cluster around the mean in a normal distribution. In fact:
  • 68% of values fall within 1 standard deviation of the mean
  • 95% fall within 2 standard deviations of the mean
  • 99% fall within  3 standard deviations of the mean
It's called the empirical rule since experimenters have observed roughly these patterns from their data over and over again when they empirically collect data.

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