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When are equal-weighted index funds / ETFs preferable to market-cap-weighted funds?
Equal-weight ETFs remove the large cap bias found in most popular indexes. What results behaves very much like a small-cap or mid-cap index. Observe RSP vs IJR over a 5 year period: IJR (iShares S&P SmallCap 600 ETF) vs RSP (Rydex S&P Equal Weight ETF)  I'm not sure if equal-weighting is worth the reduced efficiency. Mid-cap and small-cap funds have lower expenses (%0.20 for IJR vs %0.40 for RSP) and appear to do better over the long run. We don't know if that pattern will continue, but expense is one of the strongest long-term predictors of performance.