Correlation coefficient. A measure of the linear relationship
between two variables. This is a unitless value.
In the R docs, var is listed on the same page as var
(variance) and cov
(covariance).
When you want to try and predict the effect an x value has
on a y value, your calculate correlation (usually represented with 'r').
When r is greater than zero, it indicates a positive
correlation, and when less than zero, it indicates a negative correlation. An r
of zero means no correlation.
Correlation is a linear measure. Therefore you can only use
it when a plot of the x/y relationship is more linear than otherwise.
R Example
> duration = faithful$eruptions # eruption durations
> waiting = faithful$waiting # the waiting period
> cor(duration, waiting) # apply the cor function
[1] 0.90081
The value of r is
nearly 1 (one), a positive correlation.
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