This is a very basic question to ask, but I'm not sure I actually understand some fundamental properties people seem to ascribe to correlation.
As far as I understand it, correlation usually refers to Pearson's correlation coefficient, which is, according to Wikipedia, "a correlation coefficient that measures linear correlation between two sets of data." Cool.
But then I see a discussion on whether variables X and Y are related in some way, and reads like:
- well, X correlates to Y with r=0.8 so it's a good predictor/proxy/whatever
- X and Y have been found to correlate with r = 0.05, so its not a good predictor/proxy/whatever
For the first one, I'm OK. For the second one, not so... (read 211 more words →)