code: function sample_correlation(x, y) {
          var cov = sample_covariance(x, y),
              xstd = sample_standard_deviation(x),
              ystd = sample_standard_deviation(y);
  
          if (cov === null || xstd === null || ystd === null) {
              return null;
          }
  
          return cov / xstd / ystd;
      }
docs: # [correlation](http://en.wikipedia.org/wiki/Correlation_and_dependence)  Gets a measure of how correlated two datasets are, between -1 and 1  depends on `sample_standard_deviation()` and `sample_covariance()`