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