I have no doubt that the valuation tool most widely used by individual investors is the price-to-earnings ratio (P/E). Unfortunately, it may also be the most dangerous tool, because it's so misunderstood. Today, I'll talk (well, type) a little about what the P/E's problems are and how you can overcome them.

What it is
On the surface, this is a very simple and informative ratio -- a company's stock price divided by its last 12 months of earnings per share. So we can look and see that General Electric (NYSE: GE) earned $1.28 per share over the past year. At today's price of $16.60, its P/E ratio is $16.60 / $1.28 = 13. You might also hear the hip Wall Street crowd say "GE has a multiple of 13" -- because it sounds so cool.

Because you obviously want more earnings for every dollar you invest, a lower P/E is considered more attractive. After all, you'd rather be paying $8.30 per share for GE for its $1.28 in earnings (P/E = 6.5) than $16.60 (P/E = 13), right?

Yes, absolutely -- why wouldn't you want to pay less for the exact same earnings stream? The same principle applies when comparing different businesses with each other, as long as they are equal in all other respects.

What it isn't
Of course, things are never equal in the investing world (you didn't need me to tell you that), and this is where problems creep in. It's also why the P/E ratio should never be the only tool you use to value a business, for several reasons. Let's look at three of them.

1. Forward to the future
A glance at most any financial data provider tells us that Monsanto (NYSE: MON) is trading at a P/E of 20. The wonderfully named Potash Corp. of Saskatchewan (NYSE: POT) has a multiple of 11. So Potash must be a better value, right?

Well ... maybe. It's rare to find two businesses that are exactly alike, and these two, while in the same industry, certainly have many differences. Also, what about earnings in the years ahead? If Monsanto is able to rake in more cash than PotashCorp. in the coming decades, wouldn't that make up for the difference in the multiples?

So there you run into a big problem with the P/E: It's a short-sighted, usually backward-looking tool. If one company is able to double its earnings in a few short years while another remains stagnant, the former could be a much better value despite a higher multiple. Yet you wouldn't know it from the single-snapshot picture the P/E provides.

The "forward P/E" published by some sources is a better tool, because it uses the next year's estimated earnings for the "E" part of the equation, instead of the previous year's earnings. But that still provides only a very limited snapshot.

Company

Estimated 2-Year Growth Rate

Trailing P/E

Forward P/E*

Newmont Mining (NYSE: NEM)

36%

37

19

Intel (Nasdaq: INTC)

33%

45

17

EMC (NYSE: EMC)

34%

31

18

priceline.com (Nasdaq: PCLN)

49%

35

21

Data provided by Capital IQ (a division of Standard & Poor's).
*Using next 12 months' earnings estimate.

These multiples are all over the place. priceline looks like a bargain, but can it really grow at nearly 50% per year? Even Intel's and EMC's 33% growth rates are tough to achieve.

Newmont Mining provides what many view as a classic example of a fairly valued company: P/E about equal to its growth rate, with the forward P/E quite a bit lower than the trailing multiple. But if you look back at the last three full years, the company earned $1.75,  ($4.17), and $1.88 per share. What kind of confidence can you have in future earnings predictions?

So many variables!

2. Focus on fundamentals
As the previous example shows, the P/E becomes more useful if you can get a grasp on just how much in earnings a company will be able to achieve over the coming years. But in order to do this, you'll need to study the underlying business and understand its margins, scalability, competitive position within the industry, etc. This fundamental analysis goes a long way toward putting the P/E in its proper perspective.

3. The fiction of accounting
Another problem comes in defining "earnings," the denominator in our equation. Like Donald Trump's hair color, it isn't always what it seems. In fact, unlike Donald's hair, it's so easy to massage and manipulate the earnings number that it's a wonder the Street still hangs on every penny. While one company may report a largely honest number, its competitor may be padding the EPS in order to "meet estimates." In the end, stuff like that usually catches up to these companies -- and, in turn, their shareholders.

My story
The best way to think about the P/E ratio reinforces what Motley Fool co-founders Tom and David Gardner tell members of their Motley Fool Stock Advisor investing service: You must understand that you're buying a business when you buy a stock. I'd heard that often enough, but it took a real-life example to drive the point home.

Several years ago, a friend and I considered buying a sporting goods store. It was a family run business, with virtually no debt -- or cash -- on the balance sheet. The owner wanted $100,000 for it.

What's the most important thing you'd want to know if you'd been in our situation? Right: How much money we could reasonably expect to earn over the next few years. We didn't care about the fiction of accounting earnings (point No. 3 above) -- we wanted to know how much in real cash profits the business could pull in.

If it were $10,000 per year, we'd be getting the company at a P/E of 10 ($100,000 / $10,000 = 10). If it were $5,000, that's a much less attractive multiple of 20.

We dived into the fundamental analysis to figure out the real earning power of the company (point No. 2). The short story is, we thought we could make the business much more efficient (improving the margins), but future revenue and earnings growth (point No. 1) seemed very limited due to several well-financed competitors in the area.

In assessing whether this was a smart purchase, we used the three points to come up with our decision. In the end, we passed on the purchase because the P/E we calculated was over 20. Using that number hand-in-hand with our analysis, we knew it would have taken too long for us to even recoup our original investment. But the exercise itself was a lesson I'll never forget.

The lesson
Using P/E as a stand-alone valuation tool could cost you big-time. Isolating on any single metric, for that matter, is a recipe for disaster.

When David recommended Amazon.com in September 2002, he acknowledged that its P/E of 75 carried risks. But his fundamental analysis convinced him the company deserved that multiple, and the stock is up more than 480% since.

If you'd like a look at all of the Gardners' recommendations, which are beating the market by an average of 40 percentage points each, consider a 30-day free trial. This also includes the top five stocks for new money now. For more information, click here.

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This article was originally published June 19, 2009 as "Why the P/E Ratio Is Dangerous." It has been updated.

Rex Moore is a Fool analyst who's been cleared to operate heavy machinery. He owns no companies mentioned in this article. Amazon.com and priceline are Motley Fool Stock Advisor selections. Intel is an Inside Value recommendation. The Fool has a disclosure policy.