Testing relationships between data streams
In order to have correlation defined, you must understand that it is a comparison. It is a comparison between two datasets, distributions, lines or curves. The comparison will tell you how alike or unalike they are. It will even tell you if they have a negative relationship with each other. Correlation's strength is its ability to not only find black and white relationships, but gray ones as well.
The sample correlation coefficient, r has a range of -1 to 1. In general, the meaning is:
Of course there are all decimal points between these numbers (the gray). You are basically looking for an r greater than .7 to say that you have a high correlation between the 2 entities.
The sample correlation coefficient is obtained using this equation:
If you go to Simple Regression Calculation, you can see how correlation is used. In this case, it tells you how well the regression line fits the data from which it is derived.
With correlation defined, you have discovered that it is for comparison. It provides a measure of how well similar sets of data are related to each other.
The links below are specific questions and answers about statistics and how to use them.