Calculate Standard Deviation

public static double GetStandardDeviation(this IEnumerable<int> values)
{
    double standardDeviation = 0;
    int[] enumerable = values as int[] ?? values.ToArray();

    int count = enumerable.Count();
    if (count > 1)
    {
        double avg = enumerable.Average();
        double sum = enumerable.Sum(d => (d - avg) * (d - avg));
        standardDeviation = Math.Sqrt(sum / count);
    }

    return standardDeviation;
}

// Usage
List<int> ints = new List<int>() { 4, 7, 3, 9, 13, 90, 5, 25, 13, 65, 34, 76, 54, 12, 51, 23, 3, 1, 7 };
double stdev = ints.GetStandardDeviation();