### Half Normal Distribution

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The half normal distribution occurs when measurements of a normally distributed process are folded about the mean in the positive direction. For example, if a process is normally distributed with a mean 0, but the measurements can only be reported as positive values (i.e. a measurement of -0.002 is reported as +0.002), the observed distribution would be half normal. The half normal distribution has two parameters. The parameter a is the folding point (mean of the associated normal distribution). The parameter `sigma` is a scale parameter (standard deviation of the associated normal distribution). The probability density and distribution functions are given by:

`f(x) = 2/(sigma sqrt(2pi)) e^((-(x-a)^2)/(2sigma^2))`

`F(x) = 2Phi((x-a)/sigma) -1`