C# Program to Find the Standard Deviation

/*
 * C# Program to Find the Standard Deviation of a Set of Given Numbers
 */
using System;
using System.Collections.Generic;
namespace SampleApp
{
    internal class Program
    {
        private static void Main()
        {
            List<double> number = new List<double> { 1, 2, 3, 4, 5, 6 };
 
            double mean = number.Mean();
            double variance = number.Variance();
            double sd = number.StandardDeviation();
 
            Console.WriteLine("Mean: {0}  , Variance: {1}  , SD: {2}  ", 
                               mean, variance, sd);
            Console.ReadKey();
        }
    }
    public static class list
    {
        public static double Mean(this List<double> values)
        {
            return values.Count == 0 ? 0 : values.Mean(0, values.Count);
        }
 
        public static double Mean(this List<double> values, 
                                  int start, int end)
        {
            double s = 0;
 
            for (int i = start; i < end; i++)
            {
                s += values[i];
            }
 
            return s / (end - start);
        }
 
        public static double Variance(this List<double> values)
        {
            return values.Variance(values.Mean(), 0, values.Count);
        }
 
        public static double Variance(this List<double> values, double mean)
        {
            return values.Variance(mean, 0, values.Count);
        }
 
        public static double Variance(this List<double> values, double mean, 
                                      int start, int end)
        {
            double variance = 0;
 
            for (int i = start; i < end; i++)
            {
                variance += Math.Pow((values[i] - mean), 2);
            }
 
            int n = end - start;
            if (start > 0) n -= 1;
 
            return variance / (n);
        }
        public static double StandardDeviation(this List<double> values)
        {
            return values.Count == 0 ? 0 : values.StandardDeviation(0, values.Count);
        }
        public static double StandardDeviation(this List<double> values, 
                                               int start, int end)
        {
            double mean = values.Mean(start, end);
            double variance = values.Variance(mean, start, end);
            return Math.Sqrt(variance);
        }
    }
}