Here's an example of three core, diversified dividend funds that have had good gains this year through April 30, 2016. These ETFs sound the same, but you can see that they are invested in different areas of the market.

iShares High Dividend ( HDV)  is almost entirely invested in the largest stocks, while iShares Select Dividend and SPDR S&P Dividend ( SDY)  have more exposure to mid- and small-caps. iShares Select Dividend is overweight in utilities, a sector that's not in the top three for SDY or HDV. SDY has more exposure to industrials than either of the other two ETFs, and HDV has more invested in energy stocks.

If you bought all three of these ETFs, you'd essentially own about 217 individual stocks, and only eight of these stocks were held by all three funds. The majority of the stocks—74%—were owned by just one of the ETFs.Funds that have done well recently, like these dividend ETFs, tend to do well for a few months or even a few years (academics call this 'persistence of performance'). But that doesn't mean every strong-performing ETF will continue to excel. One of these dividend ETFs will likely stumble before the others, and by investing in several strong performing dividend funds, investors can increase their chances of owning an ETF that continues to outperform.

This article is commentary by an independent contributor. At the time of publication, the author had positions DVY, SDY and HDV.

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