One of the most common metrics used to gauge a stock's value is its price-to-earnings (P/E) ratio. It's the multiple of earnings you're willing to pay for a business -- the current price divided by the company's earnings per share. It's a good way to get a rough idea of whether a stock is cheap or expensive, but Fools shouldn't rely entirely on the P/E for their value assessments.

How the P/E works
Amazon.com
(NASDAQ:AMZN) has a P/E ratio of about 50, implying that investors are currently willing to pay $50 for every dollar of earnings Amazon generates. Similarly, Best Buy's (NYSE:BBY) P/E ratio of around 14.5 implies that investors are currently willing to pay $14.50 for each buck the electronics retailer makes.

For a different perspective, try flipping the P/E ratio to an E/P ratio, commonly referred to as the earnings yield. Like a yield on a bond, this number shows a company's annual earnings as a proportion of its market value. Buying one share of Amazon at $77, with EPS of $1.56, equates to an earnings yield of approximately 2% ($1.56 divided by $77). For Best Buy, the earnings yield is 6.8%, because each share of Best Buy stock, currently trading around $35, is expected to generate $2.39 in earnings per share. In the long run, Best Buy investors theoretically should earn a better rate of return than Amazon investors for each dollar invested.

Yet over the past three years, Best Buy has seen its stock lose an average of 11% annually, while Amazon's stock has jumped 29% per year. Of course, this is a very extreme, and to some degree unfair, comparison, since the two companies have very different business models. It's an apples-to-oranges comparison between an online retailer versus a big-box store. Like Amazon, Best Buy's stock-price performance was downright astonishing during its early years. So, what gives?

Avoid the pitfalls
Does the difference in P/E ratios alone make one company a better investment than the other? Not really. Although useful, P/E ratios have limitations. Yet investors tend to rely excessively on this one variable in determining an investment's attractiveness.

Most investors tend to agree that in general, businesses with lower P/E ratios tend to outperform high-P/E-ratio companies in the long run. However, that premise alone does not imply that all companies sporting single-digit ratios are superior investments. Just look at Citigroup (NYSE:C) over the past couple of years. Before the financial crisis hammered the company, Citigroup tended to have P/E ratios in the low teens. But those who bought in then are now sitting on big losses. At best, Citigroup faces an uncertain future, and its P/E ratio won't mean much for the foreseeable future.

Quality, not quantity
Fundamental problems exist with the P/E ratio. First, the "P" only refers to the equity price of a business; it doesn't consider debt. That's fine for companies without debt, like Apple (NASDAQ:AAPL) or Research In Motion (NASDAQ:RIMM), but it's meaningless when there is debt involved. For instance, a business with a market cap of $5 billion, with $1 billion of net debt on the balance sheet, has an economic (or enterprise) value of $6 billion. If this company earns $500 million in profit in a given year, the P/E ratio would be 10, but in reality, investors should see it as 12. That can make a big difference in companies with huge amounts of debt, such as General Motors (NYSE:GM) and Ford (NYSE:F).

The "E," or the per-share net income of a company, also poses problems. Creative accounting decisions, such as changing depreciation schedules or including non-recurring gains at certain points in time, can manipulate this figure. We all know that management has a considerable incentive to meet earnings expectations, so you should always examine earnings with a healthy dose of skepticism.

Most investors employ a host of investment considerations when assessing the value of a business. While the P/E ratio is a very useful resource, its often-ignored limitations can sometimes catch smart investors off guard. Use it carefully and prudently, and you'll be a lot less likely to fall into costly situations.

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This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.