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, SEI Investments (Nasdaq: SEIC).

SEI Investments shares have simply crushed the S&P 500 over the last quarter-century, with most of the outperformance occurring in the last 12 years:

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Source: S&P Capital IQ.

Since 1987, shares have returned an average of 17.1% a year, compared with 9.7% a year for the S&P (both include dividends). That difference adds up fast. One thousand dollars invested in the S&P in 1987 would be worth $19,200 today. In SEI Investments, it'd be worth $158,200.

Dividends accounted for a lot of those gains. Compounded since 1987, dividends have made up about 20% of SEI Investments' total returns. For the S&P, dividends account for 39% of total returns.

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

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Source: S&P Capital IQ.

Again, big outperformance. Since 1995, SEI Investments earnings per share have increased by 16.1% per year, compared with 6% a year for the broader index.

What's that meant for valuations? SEI Investments has traded for an average of 26 times earnings since 1987 -- a bit above the 24 times earnings of the broader S&P 500.

Through it all, shares have been strong performers 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 SEI Investments with a three-star rating (out of five). Care to disagree? Leave your thoughts in the comment section below, or add SEI Investments to My Watchlist.