The correlation between two random variables is a measure of how closely they relate. It may be a positive correlation if they increase together or a negative correlation of one goes down as the other goes up. The term is sometimes used in this general way, but in statistics it refers to the linear relationshiop between variables, that is the extent to which two variables, say x and y, lie close to a line y=mx+c for some values of m and c.
Used in Chap. 12: page 185
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