/// <summary>
/// Evaluates the population covariance from the provided full populations.
/// On a dataset of size N will use an N normalizer and would thus be biased if applied to a subset.
/// Returns NaN if data is empty or if any entry is NaN.
/// </summary>
/// <param name="population1">The full population data.</param>
/// <param name="population2">The full population data.</param>
public static double PopulationCovariance(this IEnumerable <double> population1, IEnumerable <double> population2)
{
var array1 = population1 as double[];
var array2 = population2 as double[];
return(array1 != null && array2 != null
? ArrayStatistics.PopulationCovariance(array1, array2)
: StreamingStatistics.PopulationCovariance(population1, population2));
}