a relationship between two variables, x and y, in which the change in value of one variable is exactly proportional to the change in value of the other. That is, knowing the value of one variable exactly predicts the value of the other variable (i.e., rxy = 1.0). When graphed, a perfect correlation forms a perfectly straight line. If the variables change in the same direction (i.e., they both increase or both decrease), the correlation is perfect positive, whereas if the variables change in opposite directions (i.e., one increases as the other decreases or vice versa), the correlation is perfect negative. It is uncommon to observe this type of relationship in actual data. See also correlation coefficient.