As many of you who've met me know, I'm a statistics whore, and I calculate correlations on everything. Well, if you're wondering about correlations, wonder no more!

If the correlation between two variables X and Y is 1, then increasing the value of X always leads to a proportional increase in Y. If the correlation is .5, then there is a 50% chance of an increase in X leading to a proportional increase in Y. If the correlation is 0 then the data is random. For example, there is probably a correlation of 1.0 between earning 100k+ per year and driving a Ferrari, but probably a 0 correlation between wearing glasses and driving a Ferrari.

It is often said that "Correlation is not causality", but this isn't quite true. Correlation is a necessary, but not sufficient precondition for causation. If a correlation exists between A and B, there are 3 possibilities:

A causes B
B causes A
A and B share a common cause

Note that for there to be any of the relationships shown above between two variables, there MUST be a correlation.