The correlation coefficient r measures how tightly the dots of a scatter plot follow a straight line. It runs from -1 to 1: its sign follows the slope, and its size shows the strength, with 0.7 and above strong and under 0.3 little or none. The formula averages the products of the x and y z-scores. A strong r does not prove that x causes y: a lurking variable may drive both, and only a randomized experiment can show a cause.