Growth is good.

While that's common wisdom in the stock market, the price you pay for that growth also matters. A while back, we took a look at the above truism in the context of the venerable PEG ratio. Today, we're going to look at the other half of the equation: the P/E. Or more specifically, the "E."

In his Investment Fables class, Professor Aswath Damodaran of the Stern School of Business at New York University explains the risks inherent in "choosing an 'E.'"

He writes, "The biggest problem with P/E ratios is the variations on earnings per share used in computing the multiple. The most common measure of the P/E ratio divides the current price by the earnings per share in the most recent financial year. ... Other people prefer to ... add up the earnings per share ion each of the last four quarters. ... Some analysts go even further and use expected earnings per share in the next financial year."

If you ever wondered why different websites give you different P/Es for the same company, then congratulations -- you've found the root of the problem. As I type this column, the markets are closed, and the stock price of Oracle (NASDAQ:ORCL) is static. Yet Google Finance tells me the stock trades for a 23.28 P/E. Yahoo! Finance says, no, it's 24.95. And MSN Money comes up with 23.30. One number is the "current P/E," the next is "trailing," and the third is anybody's guess.

Granted, these contradictions are more a nuisance than a danger to your portfolio. But hold on a second, I'm getting to the dangerous part. The trouble comes when investors don't like what they see in a company's current or trailing earnings, and instead choose to calculate a "forward P/E" -- the "expected earnings per share in the next financial year" that Damodaran speaks of.

A hypothetical based on a hypothetical
Deciding that a stock is fairly priced by assuming an unrealistic and analyst-hyped growth rate is dangerous enough. But you can compound the danger by compounding your assumptions, because analyst estimates of future growth are just that: Estimates. Assumptions. Guesses.

You see, your standard PEG ratio is at least partially based on fact. The "P" is fact; it's the price you see quoted today. The "E" has already happened, so that number is trustworthy (it's last year's recorded earnings, or the most recent four quarters' worth of earnings, so you can't really argue with it).

But if instead of basing your "E" on facts, you use an "E" that an analyst has postulated for next year's earnings, then your PEG ratio is constructed from just one fact (the price) and two guesses (the analyst's guess as to what next year's earnings will be, and the analyst's guess as to how fast the company will grow during the next five years). Thus, you're calculating your PEG by taking a hypothetical P/E and dividing it by a hypothetical G.

If you still don't think this is a problem, let's go to the videotape. OK, I don't have videotape, I'm just a sucker for a sports reference. Instead, let's look at a few statistics for several companies in the cyclical retail industry:

Company

Trailing P/E

Forward P/E

Growth estimate

Abercrombie & Fitch (NYSE:ANF)

15.1

11.4

15%

American Eagle (NASDAQ:AEOS)

17.2

14.0

15%

bebe stores (NASDAQ:BEBE)

20.2

16.6

20%

Volcom (NASDAQ:VLCM)

27.5

19.6

25%

Urban Outfitters (NASDAQ:URBN)

22.6

15.0

25%

Zumiez (NASDAQ:ZUMZ)

65.2

35.1

30%

All data courtesy of Yahoo! Finance.

Now look at how these companies' PEGs work out when based mostly on facts (trailing P/Es), as opposed to fiction (forward P/Es):

PEG based on Trailing P/E

PEG based on Forward P/E

Abercrombie

1.00

0.76

American Eagle

1.15

0.93

bebe

1.01

0.83

Volcom

1.10

0.78

Urban Outfitters

0.90

0.60

Zumiez

2.17

1.17

Analyst estimates -- flawed and overoptimistic as they might be -- at least take today's earnings numbers and extrapolate out how fast earnings might grow from a factual starting point. Based on those numbers, a value investor has to be leery of investing in any of the above companies at today's prices. Compound your hypotheses, however, and almost anything can start to look like a "buy."

Don't guess. Assess. As harsh as the above assessment might seem on the surface, I really don't mean to knock any of these companies. On the contrary, at the Fool's flagship newsletter, Motley Fool Stock Advisor, we have actually recommended bebe stores and American Eagle. We like the companies and we like their prospects.

All I'm trying to do today is make you more aware of the hazards of wishful thinking -- and relying too heavily on Wall Street guesswork when valuing your investments. Leave the pros to their guesswork. In your own investing, do what we do at Stock Advisor:

• Estimate reasonable rates of growth.
• Demand reasonable prices, giving yourself a wide margin of safety.
• And always, always do your due diligence before investing. There's a reason behind each and every recommendation we make.

If you want to learn why we originally picked -- and still like -- bebe and American Eagle, you can read our full write-ups on these and the 100 other recommendations we've made over the past four years, free of charge. All you have to do is take a 30-day trial of Stock Advisor. Click here to get started.

This article was originally published May 24, 2006, under the title "More Dangerous Growth." It has been updated.

Fool contributor Rich Smith does not own shares of any companies named above. If he did, he'd have to tell you so. Fool's honor. Volcom and Zumiez are Hidden Gems picks.