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In these uncertain days, bond funds are an especially important option for investors. Unlike stock funds, they have high predictability in at least these five ways: (1) The current yields (on longer-term issues) are an excellent—if imperfect—predictor of future returns. (2) The range of gross returns earned by bond managers clusters in an inevitably narrow range that is established by the current level of interest rates in each sector of the market. (3) The choices are wide. As the maturity date lengthens, volatility of principal increases, but volatility of income declines. (4) Whether taxable or municipal, bond fund returns are highly correlated with one another. Municipal bond funds are fine choices for investors in high tax brackets, and inflation-protected bond funds are a sound option for those who believe that much higher living costs will result from the huge federal government deficits of this era. (5) The greatest constant of all is that—given equivalent portfolio quality and maturity—lower costs mean higher returns. (Don’t forget that index bond funds—or their equivalent—carry the lowest costs of all.)
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John C. Bogle (Common Sense on Mutual Funds, Updated 10th Anniversary Edition)