The "Rule of 72" is a great way to calculate compounding interest in your head. The rule states that to find the number of years it would take a figure to double, simply divide the number 72 by the assumed growth rate. For example, if you think your stock will grow at a rate of 7.2% per year, it will take roughly 10 years for it to double (72/7.2 = 10).

A 7.2% annualized return doesn't sound like a great 10-year return for stocks -- and it's not. Consider that the Vanguard Total Bond Market Index (VBMFX) returned 6% per year on average over the past 10 years. A 1% risk premium for investing in stocks instead of bonds is simply not a great margin.

Yet there are stocks that return just that -- or even worse.

Consider that of the 54 companies capitalized at more than $25 billion in November 1996, more than half (30) failed to achieve a 7.2% annualized return over the past 10 years. Some of the more disappointing investments include:


10-Year Annualized Return*

Bristol-Myers Squibb (NYSE:BMY)


Coca-Cola (NYSE:KO)


DuPont (NYSE:DD)


Schering-Plough (NYSE:SGP)


And these aren't no-name stocks, either. All four are S&P 500 components. Makes you want to take a closer look at your index fund, doesn't it?

Lean on me
The good news is that for each of the aforementioned underperforming mega-caps, there have been others who've been holding down the fort over the past 10 years. Without these companies, the S&P 500 wouldn't have returned approximately 8.0% per year during that stretch. Included in this list are:


10-Year Annualized Return*

Citigroup (NYSE:C)


ExxonMobil (NYSE:XOM)


Microsoft (NASDAQ:MSFT)


Now we're talking. This is the kind of growth you expect to see from your stocks. This is why you take the extra risk by investing in stocks instead of bonds or CDs.

But is the best good enough?
Yet these were among the best-performing mega caps of the past 10 years, and I consider a 15% annualized return to be about the most any mega-cap investor can hope for. Why? Simply put: The Law of Diminishing Returns. As it becomes big, a company's growth begins to plateau. Microsoft, after all, was once a tremendous growth stock -- averaging greater than 50% annual returns from 1986 to 2000 -- before it got so big that it became more difficult for its growth efforts to really drive the bottom line.

Furthermore, according to Prof. Jeremy Siegel's research, only 11 S&P stocks were able to sustain more than 14.7% annual returns from 1957 to 2003, even after we include dividend reinvestment! That fact alone illustrates the reality that great companies eventually mature and their returns diminish.

You can do better
Small-cap stocks, on the other hand, have much more room to grow than their larger counterparts. Indeed, all of the market's 10 best stocks of the past 10 years were small caps. Of this group, Hansen Natural returned 23,300% and Chico's FAS yielded 4,696%. To put that type of growth into perspective, if you had invested $10,000 in Chico's in June 1996, you would have had $479,600 10 years later.

This isn't to say you should scrap your large caps -- diversification is important -- but if you're looking for a few great growth stocks to add your portfolio, you may want to consider a small company rather than one of the S&P's giants.

Fool's final word
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This article was originally published on Oct. 20, 2006. It has been updated.

At time of publication, Todd Wenning did not own shares of any company mentioned in this article. Coke and Microsoft are Motley Fool Inside Value picks. DuPont is an Income Investor selection. The Fool isinvestors writing for investors.