code: protected static double calcP_i(double f, double mu, double sigma) {
      final double fmu = f - mu;
      return ONEBYSQRT2PI / sigma * FastMath.exp(fmu * fmu / (-2 * sigma * sigma));
    }
docs: Compute p_i (Gaussian distribution, outliers)
  
  @param f value
  @param mu Mu parameter
  @param sigma Sigma parameter
  @return probability