calculate the standard deviation of an array of strings

public double StdDev(double[] nums)
    {
        var arr = new double[nums.Length];
        Array.Copy(nums, arr, nums.Length);
        Array.Sort(arr);
        var q1 = arr[arr.Length / 4];
        var q3 = arr[arr.Length * 3 / 4];
        var iqr = q3 - q1;
        var lb = q1 - 1.5 * iqr;
        var ub = q3 + 1.5 * iqr;
        arr = arr.Where(num => num >= lb && num <= ub).ToArray();
        var mean = arr.Average();
        var sumOfSquaresOfDifferences = arr.Select(val => (val - mean) * (val - mean)).Sum();
        var sd = Math.Sqrt(sumOfSquaresOfDifferences / arr.Length);
        return sd;
    }