"The price has dropped -- should I buy?"

That's the question ringing in your ears if you've been thinking about buying shares of Dell lately. At the beginning of June, the stock traded hands at about $26. It ended last week 23% lower.

You're also asking yourself that question if you've taken a long, hard look at Home Depot (NYSE:HD). The home-supplies leader has lost more than 17% of its value in the past two-and-a-half months.

Sure, as an investor, you know the market is a volatile place. But in the face of recent stock drops, you're undoubtedly worried that the prices of these once-great companies will continue to fall.

Too much projecting
Don't worry, you're not just a worrywart -- you're suffering from a little-known investing disease called "extrapolation fallacy." This occurs when events in the recent past are extrapolated indefinitely into the future. In other words, if Dell's stock price has fallen, you think it will always fall. Not so. I hope you can see how silly the extrapolation fallacy really is.

You're not alone in your silliness. Extrapolation fallacy is a common ailment in the market. On a broad basis, it contributes to prolonged bear markets. But it can also work in reverse. It can cause people to assume that things are great, and they'll always be great, which leads to financial bubbles -- like the dot-com craze of the late 1990s. You know, the one Alan Greenspan dubbed "irrational exuberance."

Good results in the long term
The truth is that stock prices rise and fall, but in general they do more rising than falling. Just take a look at the long-term charts of companies such as Hewlett-Packard (NYSE:HPQ) and Altria (NYSE:MO). Hewlett-Packard was trading at a split- and dividend-adjusted $1.40 or so in June 1980. As I write, it trades at about $30.50, a return of more than 2,000%. Along the way, however, it declined 50% in 1990 and 80% from 2000 to 2002. A roller-coaster ride, indeed.

Tech companies aren't alone in their ups and downs. The long-term chart of Altria Group, for instance, shows increases of several hundred percent -- and declines of more than 50% -- throughout the years. Overall, however, Altria has returned more than 13,000% since 1980.

It's said that history never repeats itself, but it sure does come close. Down trends like the one we're seeing now do not necessarily herald the end of a company. Similarly, up trends will not continue unabated. Whether the company you are interested in is a $340 billion giant like General Electric (NYSE:GE), a $9 billion mid-cap like Coventry Healthcare (NYSE:CVH), or a $750 million little guy like Take-Two Interactive Software (NASDAQ:TTWO), the same rise-and-fall patterns will emerge, but generally they end up, well up. Over the long term, with well-chosen companies, you can turn a relatively small investment into a huge fortune.

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This article was originally published on June 22, 2006. It has been updated.

Fool contributor Jim Mueller owns shares in Dell, and his wife owns some General Electric. Dell and Coventry Healthcare are Stock Advisor picks. Dell and Home Depot are Inside Value picks. The Motley Fool has a strictdisclosure policy.