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A bell curve is often
referred to as the normal distribution, with mean μ and variance
σ². A bell curve is perfectly
symmetric about the mean and its spread is measured by the standard deviation
σ. This distribution approximates
the probability distributions of numerous random variables.
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For example, variables
ranging from the outcomes of flips of a coin or the rolls of dice to the
number of grains of sand in a cubic yard to the height, weight, or
cholesterol levels of 30-year-old American males to the prices of gasoline in
North Carolina to all tend to be normally distributed and describable by a
bell curve.
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This
table identifies the percentage of the events expected to fall around the
estimated mean of a variable with a normal probability distribution (a bell
curve), within plus or minus the specified numbers of standard deviations.
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range
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confidence
interval
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1 σ
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68.3%
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2 σ
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95.4%
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3 σ
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99.7%
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4 σ
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99.9%
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________________________________________________________________________________________________________________________________________________
Author: Ralph Byrns
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Economics
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