Price/earnings-to-growth ratio (PEG)

To find a stock's PEG ratio, you divide the stock's price-to-earnings ratio (P/E) by its projected annual earnings per share (EPS) growth. The result is a rule-of-thumb assessment of whether the stock is overvalued or undervalued.

In brief, if a stock has a PEG ratio of 1, you conclude that investors are paying what the stock is worth based on its P/E and growth potential. If it is higher than 1 -- say 1.55 -- you conclude that investors are paying more than the growth projection justifies. If it is less than 1, you conclude that the stock may be poised to appreciate in value and so a wise purchase.

However, a PEG ratio, by itself, does not provide an adequate basis for an investment decision, any more than a P/E does, because it doesn't take company fundamentals into account. For example, an under-priced stock may be a good buy, but it may also be the sign of a company in poor financial shape or an industry in trouble.

The potentially greater problem is that growth projections, even when they are the consensus finds of professional analysts, are just estimates. That is especially true of estimates that look out five or more years, since there is no way to anticipate the shifting marketplace with real precision. Yet projections based on a single-year's results are notoriously inaccurate.

In short, a PEG ration can be a valuable addition to an investor's toolkit, provided you understand the assumptions on which its components and results are based.