One of the most important aspects of the S 500 index is the way it is weighted. While its market-cap weighting approach was an improvement over the way the Dow Jones Industrial Average was designed, it exposes the S 500 index -- and any products that track it -- to extra risk during periods of excess.

That's why adding the Invesco S 500 Equal Weight ETF (NYSEMKT: RSP) to your portfolio might be a no-brainer move today if you have $1,000 in cash available to invest.

When the Dow Jones Industrial Average was created, people had to do math by hand.

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Source Fool.com