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. More specifically, the "E."

In his Investment Fables class, Prof. 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 in each of the last four quarters. ... Some analysts go even further and use expected earnings per share in the next financial year.

If you've 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 Merck (NYSE:MRK) is static. Yet Google Finance tells me the stock trades for a 17.6 P/E. Yahoo! Finance says no, it's 17.5. And MSN Money comes up with 17.9. One number is the "forward P/E," the next is "trailing," and the third is another "forward" estimate.

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 focus on 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 Merck and some other pharmaceutical and biotech companies:

Company

Trailing P/E

Forward P/E

Growth Estimate

Amgen (NASDAQ:AMGN)

31.9

17.0

15.5%

Genentech (NYSE:DNA)

48.6

31.6

29.5%

Gilead Sciences

33.1

28.5

18.5%

Merck

17.5

18.2

5.5%

Pfizer (NYSE:PFE)

18.8

13.1

3.4%

Serono (NYSE:SRA)

41.3

17.5

18.0%

Teva Pharmaceutical (NASDAQ:TEVA)

396.9

15.0

16.0%

All data from 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).

Company

PEG Based
on Trailing P/E

PEG Based
on Forward P/E

Amgen

2.1

1.1

Genentech

1.6

1.1

Gilead Sciences

1.8

1.5

Merck

3.2

3.3

Pfizer

5.5

3.9

Serono

2.3

0.9

Teva Pharmaceutical

24.8

0.9



Analyst estimates -- flawed and overly optimistic as they might be -- at least take today's earnings numbers and extrapolate how quickly earnings might grow from a factual starting point. Compound your hypotheses, however, and a few stocks can start to look like real "buys."

Don't guess. Assess.

All I'm trying to do today is make you more aware of the hazards of wishful thinking -- and of 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 our flagship Stock Advisor investing service:

  • 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'd like to read two of Fool co-founders David and Tom Gardner's picks right now, grab a copy of our new special report free of charge. All you have to do is click here and you'll get two great stock ideas sent straight to your email.

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 in any of the companies mentioned in this article. Pfizer is an Inside Value selection.Check out the Fool'sdisclosure policy.