Correlation


What does correlation measure? There is no quick answer. Typically it is taken to measure "strength of linearity." That is: given data in a scatterplot, how close is it to a line. However, there are many caveats:

The value of the correlation coefficient depends on the nature of the variability of the variables of interest. Not just the residual variability (error), but the marginal variability as well.

Comparing correlations--particularly values that are close--from unrelated studies is unwarranted. (A deeper analysis is required.)

A typical "interpretation" of correlation involves its square, r2, the coefficient of determination. Fair enough, in the sense that this value actually measures something. However, what it measures is not consistent with the units formulated in the data.

When data in two variables is averaged, the resulting correlation is larger than it is for the original data.

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