Definition:-The empirical rule is an important rule of thumb that is used to state the approximate percentage of values that lie within a given number of standard deviations from the mean of a set of data if the data are normally distributed.
The empirical rule is used only for three numbers of standard deviations 1σ,2σ, and 3σ.
Distance from the mean |
Values within Distance |
µ±1σ |
68% |
µ±2σ |
95% |
µ±3σ |
99.7% |
If a set of data is normally distributed, or bell shaped
· approximately 68% of the data values are within one standard deviation of the mean
· 95% are within two standard deviations, and
· Almost 100% are within three standard deviation.
Example:- Support a recent report states that for California, the average statewide price of a gallon of regular gasoline was $1.42. Suppose regular gasoline price varied across the state with a standard deviation of $0.08 and were normally distributed.
According to empirical rule,
Approximately 68% of the prices should fall within
µ±1σ= 1.42 ±1*0.08
= (1.42- 0.08, 1.42+0.08)
= (1.34, 1.5)
Similarly 95% of the prices should fall within
µ±2σ= 1.42 ±2*0.08
=1.42±0.16
= (1.42- 0.16, 1.42+0.16)
= (1.26, 1.58)
And 99.7% of the prices should fall within
µ±3σ= 1.42 ±3*0.08
=1.42±0.24
= (1.42- 0.24, 1.42+0.24)
= (1.18, 1.66)