code: public static function CONFIDENCE($alpha, $stdDev, $size)
      {
          $alpha = Functions::flattenSingleValue($alpha);
          $stdDev = Functions::flattenSingleValue($stdDev);
          $size = floor(Functions::flattenSingleValue($size));
  
          if ((is_numeric($alpha)) && (is_numeric($stdDev)) && (is_numeric($size))) {
              if (($alpha <= 0) || ($alpha >= 1)) {
                  return Functions::NAN();
              }
              if (($stdDev <= 0) || ($size < 1)) {
                  return Functions::NAN();
              }
  
              return self::NORMSINV(1 - $alpha / 2) * $stdDev / sqrt($size);
          }
  
          return Functions::VALUE();
      }
docs: CONFIDENCE.
  
  Returns the confidence interval for a population mean
  
  @param float $alpha
  @param float $stdDev Standard Deviation
  @param float $size
  
  @return float