LONDON -- If the long-run return on the market is 9.4% (as researchers at Credit Suisse say), investing in shares should be a no-brainer. Somehow, however, all too often our portfolios don't seem to reflect that attractive performance.

This is partly because that 9.4% number is an average derived from 100 years of data. Picking various time periods within that 100 years gives very different outcomes -- and the market almost never actually returns 9.4% in any single year.

Needless to say, unless you're holding a market tracker, your portfolio could have dramatically different results than what the market experiences. If you own a disproportionate amount of winning shares, your returns could be significantly better than the market. On the other hand...

In this series of articles, I'm looking at how individual shares have performed against the FTSE 100 (FTSEINDICES:^FTSE) during the past 10 years. Today, I'm assessing high-street retailer Marks & Spencer (LSE:MKS).

Over the last decade, Marks & Spencer's performance has failed to keep pace with that of the FTSE 100.

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Source: S&P Capital IQ.

While 2012 has been a good year for Marks & Spencer, with the shares up almost 24%, over the past 10 years, the story hasn't been quite as positive. Since December 2002 Marks & Spencer shares have returned an average of 1.9% per year, well below the 7.9% return on the FTSE 100 (these returns assume dividends were reinvested). As you can see from the chart above, the ride hasn't been smooth and there were plenty of opportunities for investors to make good money on these shares -- as long as you picked them up when they were on sale.

Over the past decade the shares have traded at an average P/E ratio of 12.8, which is slightly below the 13.1 average for the FTSE 100, but there were times when the rating was well below average.

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Source: S&P Capital IQ and Thomson Reuters.

This year's run puts the shares at just about the 10-year average P/E so investors need to ask themselves where they see Marks & Spencer going from here. Can the company find a way to grow sales ahead of the sub-3% rate we've seen over the past five years or somehow improve its margins in an environment of seriously pressured consumers?

The current yield of 4.3% is pretty attractive in today's rate environment and it doesn't appear the company's cash flows are under threat despite the tough retail market. Marks & Spencer has delivered dividend growth of nearly 6% over the past 10 years, so there is an argument to be made for holding on to the share for the income it provides, but I think I might put off buying until the price is a bit more attractive.

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Nate Weisshaar does not own any share mentioned in this article. The Motley Fool has no positions in the stocks mentioned above. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.