Correlation is a statistical measure that can show whether and how strongly pairs of variables are related. For example, height and weight are related; taller people tend to be heavier than shorter people. The relationship isn’t perfect. People of the same height vary in weight, and you can easily think of two people you know where the shorter one is heavier than the taller one. Nonetheless, the average weight of people 5’5” is less than the average weight of people 5’6”, and their average weight is less than that of people 5’7”, etc. Correlation does not imply causation, rather it implies an association between two variables. The strength of a correlation can be indicated by the correlation coefficient.
The correlation coefficient is a measure of how closely two variables move in relation to one another. If one variable goes up by a certain amount, the correlation coefficient indicates which way the other variable moves and by how much. A correlation coefficient is in the range of -1 to zero to +1. When two variables, X and Y, have a correlation coefficient approaching -1, if variable X goes up by one unit, variable Y will go down by one unit. If their correlation coefficient approaches +1, and X goes up by one unit, Y will also go up one unit. A correlation coefficient of zero means movement of the X and Y variables is unrelated.