University of California, Berkeley: Philip Stark's SticiGui: "Chapter 7: Correlation and Association"
Read this chapter for a discussion on the difference between correlation and association between two variables. When two variables are said to be highly correlated, it does not mean that one is causing the other. Pay special attention to the formula for computing the correlation value. There are several exercises and Java applets embedded in the text that are meant to further reinforce your learning. Do not skip these exercises! For instructions on how to navigate these exercises, see "Special Instructions on SticiGui Exercises and Java Applets” in the "Course Information” section above.