Investing isn't easy. Even Warren Buffett counsels that most investors should invest in a low-cost index like the S&P 500. That way, "you'll be buying into a wonderful industry, which in effect is all of American industry," he says.

But there are, of course, companies whose long-term fortunes differ substantially from the index. In this series, we look at how individual stocks have performed against the broad S&P 500.

Step on up, UnitedHealth (NYSE: UNH).

UnitedHealth shares have simply demolished the S&P 500 over the last quarter-century:

Source: S&P Capital IQ.

Since 1987, shares have returned an average of 24.1% a year, compared with 9.7% a year for the S&P (both include dividends). That difference piles up fast. One thousand dollars invested in the S&P in 1987 would be worth $19,200 today. In UnitedHealth, it'd be worth $990,300.

Dividends accounted for some of those gains. Compounded since 1987, dividends have made up about 5% of UnitedHealth's total returns. For the S&P, dividends account for 39% of total returns.

Now have a look at how UnitedHealth earnings compare with S&P 500 earnings:

Source: S&P Capital IQ.

Huge outperformance. Since 1995, earnings per share have grown by an average of 19.3% a year, compared with 6% annual growth for the broader index.

What's that meant for valuations? UnitedHealth has traded for an average of 19.7 times earnings since 1987 -- a bit below the 24 times earnings average of the S&P 500.

Through it all, shares have been massive outperformers over the last quarter-century.  

Of course, the important question is whether that will continue. That's where you come in. Our CAPS community currently ranks UnitedHealth with a five-star rating (out of five). Do you disagree? Leave your thoughts in the comment section below, or add UnitedHealth to My Watchlist.