Overlapping area represents the amount of risk that can be diversified away by holding ATT and American Airlines Group Inc in the same portfolio assuming nothing else is changed. The correlation between historical prices or returns on American Airlines Group and ATT 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 ATT are associated (or correlated) with American Airlines. Values of the correlation coefficient range from -1 to +1, where. The correlation of zero (0) is possible when the price movement of American Airlines Group has no effect on the direction of ATT i.e. ATT and American Airlines go up and down completely randomly.