R mode measures for linear environmental variables

Pearson's r
This familiar measure of linear correlation between two variables, suitable only for detecting linear relationships between variables. This is covariance between two variables divided by the product of their standard deviations. If your variables have many zeros, this correlation coefficient will not be reliable as double-zeros will be understood as an "agreement" when, in fact, they are simply the absence of an observation. This will inflate the correlation coefficient.

Covariance
The unstandardised form of linear correlation, or the covariance, between variables may also be used. Variables should be centred on their means (i.e. all variables have a mean of "0") before calculating covariances as an R-mode measure.
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