The first measure for the association of two continuous variables is covariance. Here is the formula:
Covariance indicates how two variables, X and Y, are related to each other. When large values of both variables occur together, the deviations are both positive (because Xi - Mean(X) > 0 and Yi - Mean(Y) > 0), and their product is therefore positive. Similarly, when small values occur together, the product is positive as well. When one deviation is negative and one is positive, the product is negative. This can happen when a small value of X occurs with a large value of Y and the other way ...
