According to the empirical rule, if the data form a "bell-shaped" normal distribution, percent of the observations will be contained within 2 standard deviations around the arithmetic mean.

According to empirical rule, the area under the normal distribution or "bell-shaped curve", when plotted as a function of the z-parameter can be defined in terms of percentages.
The z-parameter (or z-score) is defined as

where
x =  value of random variable,
μ = the mean,
σ = standard deviation,

The total area under the curve = 1.
The curve below shows that within 2 standard deviations from the mean, the total area under the curve is 95%  of the total area.

95%

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