/// <summary>
        /// 정규화된 제곱 평균 - Normalized root-mean-square error (RMSE) : 예측치와 실제값과의 오차를 제곱평균으로 계산하고, 정규화합니다.
        /// 참고 : http://en.wikipedia.org/wiki/Root_mean_square_error
        /// </summary>
        /// <param name="expected"></param>
        /// <param name="actual"></param>
        /// <returns></returns>
        public static decimal NormalizedRootMeanSquareError(this IEnumerable<decimal?> expected, IEnumerable<decimal?> actual) {
            decimal rmse = RootMeanSquareError(expected, actual);

            decimal min, max;
            actual.GetMinMax(out min, out max);

            return rmse / (max - min);
        }
        /// <summary>
        /// 정규화된 제곱 평균 - Normalized root-mean-square error (RMSE) : 예측치와 실제값과의 오차를 제곱평균으로 계산하고, 정규화합니다.
        /// 참고 : http://en.wikipedia.org/wiki/Root_mean_square_error
        /// </summary>
        /// <param name="expected"></param>
        /// <param name="actual"></param>
        /// <returns></returns>
        public static double NormalizedRootMeanSquareError(this IEnumerable<int?> expected, IEnumerable<int?> actual) {
            double rmse = RootMeanSquareError(expected, actual);

            int min, max;
            actual.GetMinMax(out min, out max);

            return rmse / (max - min);
        }
        /// <summary>
        /// 정규화된 제곱 평균 - Normalized root-mean-square error (RMSE) : 예측치와 실제값과의 오차를 제곱평균으로 계산하고, 정규화합니다.
        /// 참고 : http://en.wikipedia.org/wiki/Root_mean_square_error
        /// </summary>
        /// <param name="expected"></param>
        /// <param name="actual"></param>
        /// <returns></returns>
        public static float NormalizedRootMeanSquareError(this IEnumerable<float?> expected, IEnumerable<float?> actual) {
            float rmse = RootMeanSquareError(expected, actual);

            float min, max;
            actual.GetMinMax(out min, out max);

            return rmse / (max - min);
        }