Overlapping area represents the amount of risk that can be diversified away by holding IBEX 35 and ATX in the same portfolio assuming nothing else is changed. The correlation between historical prices or returns on ATX and IBEX 35 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 IBEX 35 are associated (or correlated) with ATX. Values of the correlation coefficient range from -1 to +1, where. The correlation of zero (0) is possible when the price movement of ATX has no effect on the direction of IBEX 35 i.e. IBEX 35 and ATX go up and down completely randomly.