Logistic Distribution. The Logistic distribution has density function:

f(x) = (1/b)*e-(x-a)/b * [1+e-(x-a)/b]-2

where
a     is the mean of the distribution
b     is the scale parameter
e     is the base of the natural logarithm, sometimes called Euler's e (2.71...)

The graphic above shows the changing shape of the Logistic distribution when the location parameter equals 0 and the scale parameter equals 1, 2, and 3.
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