Overlapping area represents the amount of risk that can be diversified away by holding Apple Inc and Citigroup Inc in the same portfolio assuming nothing else is changed. The correlation between historical prices or returns on Citigroup and Apple is a relative statistical measure of the degree to which these equity instruments tend to move together. The correlation coefficient measures the extent to which returns on Apple are associated (or correlated) with Citigroup. Values of the correlation coefficient range from -1 to +1, where. The correlation of zero (0) is possible when the price movement of Citigroup has no effect on the direction of Apple i.e. Apple and Citigroup go up and down completely randomly.