“Average ROI” represents the average gains that a group of companies reported after deploying a given technology. “Cumulative ROI (cROI) pretends to represent the sum of the gains made by a specific company over a certain length of time. It gives an exaggerated portrayal of returns, and gives no sense of what will happen to the bottom line when the company closes its books at the end of each year. Annual ROI and payback period together correct the faults of cumulative ROI.”]
Source: https://www.csoonline.com/article/2115904/lies–damn-lies-and-average-roi.html