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When to use geometric vs. arithmetic mean? Why is the former better for percentages?
Simple.  Say in 2012 you were up 50% (brilliant) but then in 2013 you were down 50% (sorry). i.e. if you started with $1000, you were up to $1500, then down to $750. You lost $250 overall. If you were to compute the mean of the percentages using each method, then: Arithmetic mean: The average of +50% and -50% (really 150% and 50% of each period's initial value) is zero, not up or down. Geometric mean: 1.5 * .5 = .75, i.e. you are down 25% over 2 years, or about 13.4% per year. It should be clear the Geometric makes more sense in such a case.