where r may vary from -1 to 1, with ±1 being a perfect correlation with y
increasing (+) or decreasing (-) with x, and 0 being no correlation. Special
care is required when using this parameter because it is very dependent on n,
the number of calibration samples. For example, an r value of 0.96 with 5
points is comparable to an r value of 0.56 with 20 points. It is
therefore better to use confidence levels or only compare r values for
the same number of points.