question: public static double cdf ( double x , double mu , double sigma , double skew ) { x = ( x - mu ) / sigma ; if ( Math . abs ( skew ) > 0. ) { double tmp = 1 - skew * x ; if ( tmp < 1e-15 ) { return ( skew < 0. ) ? 0. : 1. ; } x = - Math . log ( tmp ) / skew ; } return .5 + .5 * NormalDistribution . erf ( x * MathUtil . SQRTHALF ) ; }
target:  Cumulative probability density function (CDF) of a normal distribution.