Question

Does LINQ model the aggregate SQL function STDDEV() (standard deviation)?

If not, what is the simplest / best-practices way to calculate it?

Example:

  SELECT test_id, AVERAGE(result) avg, STDDEV(result) std 
    FROM tests
GROUP BY test_id

Answer 1

You can make your own extension calculating it

public static class Extensions
{
    public static double StdDev(this IEnumerable<double> values)
    {
       double ret = 0;
       int count = values.Count();
       if (count  > 1)
       {
          //Compute the Average
          double avg = values.Average();

          //Perform the Sum of (value-avg)^2
          double sum = values.Sum(d => (d - avg) * (d - avg));

          //Put it all together
          ret = Math.Sqrt(sum / count);
       }
       return ret;
    }
}

If you have a sample of the population rather than the whole population, then you should use ret = Math.Sqrt(sum / (count - 1));.

Transformed into extension from Adding Standard Deviation to LINQ by Chris Bennett.