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