Extension methods for performance statistics (avg, median, centiles)

PerfStatsExtensions.cs

using System;
using System.Linq;

namespace MyNamespace
{
    public static class PerfStatsExtensions
    {
        public static double? Avg(this double[] values)
        {
            if (values == null) return null;
            double total = 0D;
            long nb = 0L;
            foreach (double val in values)
            {
                total += val;


                nb += 1L;

            }
            if (nb > 0L)
            {
                return total / (double)nb;
            }
            return 0D;
        }
        public static double? Percentile(this double[] values, double percentile)
        {
            if (values == null) return null;

            Array.Sort(values);
            int N = values.Length;
            double n = (N - 1) * percentile + 1;
            if (n == 1d) return values[0];
            else if (n == N) return values[N - 1];
            else
            {
                int k = (int)n;
                double d = n - k;
                return values[k - 1] + d * (values[k] - values[k - 1]);
            }
        }
        public static long LessThan(this double[] values, double value)
        {
            if (values == null) return 0L;
            return values.Where(v => v < value).LongCount();
        }
        public static double PercentLessThan(this double[] values, double value)
        {
            if (values == null) return 0D;
            return 100D * values.Where(v => v <= value).LongCount()/values.Count();
        }

        //shortcuts
        public static double? _50th(this double[] values, double percentile)
        {
            return values.Percentile(0.5D);
        }
        public static double? _70th(this double[] values, double percentile)
        {
            return values.Percentile(0.7D);
        }
        public static double? _90th(this double[] values, double percentile)
        {
            return values.Percentile(0.9D);
        }
        public static double? _95th(this double[] values, double percentile)
        {
            return values.Percentile(0.95D);
        }
        public static double? _99th(this double[] values, double percentile)
        {
            return values.Percentile(0.99D);
        }
    }
}