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

That's the question ringing in your ears if you've been thinking about buying shares of Florida Rock Industries (NYSE:FRK) lately. At the beginning of May, the stock traded hands at about $65. It ended last week 40% lower.

You're also wondering if it's time to buy if you've taken a long, hard look at Cree (NASDAQ:CREE). The LED manufacturer has lost about 35% of its value since the beginning of April.

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 Cree 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 that 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 Adobe Systems (NASDAQ:ADBE) and CircuitCity (NYSE:CC).

Adobe was trading at a split-adjusted $2.08 or so in January 1993. As I write, it trades at about $38.20, a return of more than 1,730%. Along the way, however, it declined more than 50% in 1998 and mid-2002, and it fell 70% from late 2000 to early 2001. A roller-coaster ride, indeed.

Tech companies aren't alone in their ups and downs. The long-term chart of Circuit City, for instance, shows increases of several hundred percent -- and declines of more than 50% -- throughout the years. Overall, however, Circuit City has returned more than 1,200% since 1991.

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 $413 billion behemoth like ExxonMobil (NYSE:XOM), a $16 billion mid cap like Kroger (NYSE:KR), or a $526 million small cap like Jos. A Bank (NASDAQ:JOSB), the same rise-and-fall patterns will emerge, but generally they end up, well, up. Over the long term, well-chosen companies will help you turn a relatively small investment into a huge fortune.

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

Fool contributor Jim Mueller does not own shares in any company mentioned. The Motley Fool has a strict disclosure policy.