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Why use ROI if I can use effective compount interest?
Yes though I'd likely put a caveat on that. If you take short-term investments and extrapolate the results to get an annual result this can be misleading. For example, if a stock goes up 10% in a month, assuming this will continue for the next 11 months may not be a great idea. Thus, beware of how much data do you have in making these calculations. When looking at long-term investments, the compound annual growth rate can be quite useful for comparison.