Empirical Rule

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  • Post last modified:December 14, 2023
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The Empirical Rule, also known as the 68-95-99.7 rule or the Three Sigma Rule, is a statistical guideline that describes the approximate percentage of data within a certain number of standard deviations from the mean in a normal distribution. This rule is particularly applicable to data sets that exhibit a bell-shaped or symmetric distribution, such as the normal distribution.

The key components of the Empirical Rule are as follows:

1. **68% Rule:**
– In a normal distribution, approximately 68% of the data falls within one standard deviation of the mean in both directions. Mathematically, this is expressed as:
\[\text{Percentage within 1 standard deviation} = \text{Percentage within } (\mu – \sigma) \text{ to } (\mu + \sigma) \approx 68%\]

2. **95% Rule:**
– About 95% of the data falls within two standard deviations of the mean. Mathematically, this is expressed as:
\[\text{Percentage within 2 standard deviations} = \text{Percentage within } (\mu – 2\sigma) \text{ to } (\mu + 2\sigma) \approx 95%\]

3. **99.7% Rule:**
– Almost 99.7% of the data falls within three standard deviations of the mean. Mathematically, this is expressed as:
\[\text{Percentage within 3 standard deviations} = \text{Percentage within } (\mu – 3\sigma) \text{ to } (\mu + 3\sigma) \approx 99.7%\]

These rules provide a quick and practical way to assess the spread of data in a normal distribution. It’s important to note that the Empirical Rule is specific to normal distributions and may not apply to distributions with different shapes or characteristics. In practice, many real-world phenomena exhibit normal distribution characteristics, making the Empirical Rule a useful approximation.

The formulae are based on the assumption that the data is normally distributed. In a normal distribution:

– About 68% of data points fall within one standard deviation of the mean.
– About 95% fall within two standard deviations.
– About 99.7% fall within three standard deviations.

While these rules provide valuable insights, they are approximations, and deviations from normality or the presence of outliers can affect the accuracy of these percentages. Statistical tools, such as histograms, Q-Q plots, and formal tests, can be used to assess the normality of a distribution before applying the Empirical Rule.