Correlation
The correlation between two variables represents the degree to
which variables are related. Typically the
linear relationship is measured with either Pearson's
correlation or
Spearman's rho. It is important to keep in
mind that correlation does not necessarily mean causation. For
example, there is a high positive relationship between the number of
fire fighters sent to a fire and the amount of damage done. Does this
mean that the fire fighters cause the damage? Or is it more likely
that the bigger the fire, the more fire fighters are sent and the
more damage that is done. In this example, the variable "size of the
fire" is the causal variable, correlating with both the number of
fire fighters sent and the amount of damage done.