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There are multiple versions of the P–E ratio, depending on whether earnings are projected or realized, and the type of earnings.
As an example, if stock A is trading at $24 and the earnings per share for the most recent 12-month period is $3, then stock A has a P–E ratio of 24/3 or 8. Put another way, the purchaser of the stock is paying $8 for every dollar of earnings. Companies with losses (negative earnings) or no profit have an undefined P–E ratio (usually shown as "not applicable" or "N/A"); sometimes, however, a negative P–E ratio may be shown.
Some people mistakenly use the formula market capitalization / net income to calculate the P–E ratio. This formula often gives the same answer as market price / earnings per share, but if new capital has been issued it gives the wrong answer, as market capitalization = market price × current number of shares whereas earnings per share= net income / weighted average number of shares.
Variations on the standard trailing and forward P–E ratios are common. Generally, alternative P–E measures substitute different measures of earnings, such as rolling averages over longer periods of time (to attempt to "smooth" volatile or cyclical earnings, for example), or "corrected" earnings figures that exclude certain extraordinary events or one-off gains or losses. The definitions may not be standardized. For companies that are loss-making, or whose earnings are expected to change dramatically, a "primary" P–E can be used instead, based on the earnings projections made for the next years to which a discount calculation is applied.
By comparing price and earnings per share for a company, one can analyze the market's stock valuation of a company and its shares relative to the income the company is actually generating. Stocks with higher (or more certain) forecast earnings growth usually have a lower P–E, and those expected to have lower (or riskier) earnings growth usually have a higher P–E. Investors can use the P–E ratio to compare the value of stocks: if one stock has a P–E twice that of another stock, all things being equal (especially the earnings growth rate), it is a less attractive investment. Companies are rarely equal, however, and comparisons between industries, companies, and time periods may be misleading. P–E ratio in general is useful for comparing valuation of peer companies in similar sector or group.
Various interpretations of a particular P–E ratio are possible, and the historical table below is just indicative and cannot be a guide, as current P–E ratios should be compared to current real interest rates (see Fed model):
|N/A||A company with no earnings has an undefined P–E ratio. By convention, companies with losses (negative earnings) are usually treated as having an undefined P–E ratio, even though a negative P–E ratio can be mathematically determined.|
|0–10||Either the stock is undervalued, or the company's earnings are thought to be in decline. Alternatively, current earnings may be substantially above historic trends or the company may have profited from selling assets.|
|10–17||For many companies a P–E ratio in this range may be considered fair value.|
|17–25||Either the stock is overvalued or the company's earnings have increased since the last earnings figure was published. The stock may also be a growth stock with earnings expected to increase substantially in the future.|
|25+||A company whose shares have a very high P/E may have high expected future growth in earnings, or this year's earnings may be considered exceptionally low, or the stock may be the subject of a speculative bubble.|
It is usually not enough to look at the P–E ratio of one company and determine its status. Usually, an analyst looks at a company's P/E ratio compared to the industry the company is in, the sector the company is in, as well as the overall market (for example the S&P 500 if it is listed in a US exchange). Sites such as Reuters offer these comparisons in one table. Often, comparisons will also be made between quarterly and annual data. Only after a comparison with the industry, sector, and market can an analyst determine whether a P–E ratio is high or low with the above mentioned distinctions (i.e., undervaluation, over valuation, fair valuation, etc.).
Using discounted cash flow analysis, the impact of earnings growth and inflation can be evaluated. Using constant historical earnings growth rate of 3.8 and post-war S&P 500 returns of 11% (including 4% inflation) as the discount rate, the fair P/E is obtained as 14.42. A stock growing at 10% for next five years would have a fair P–E of 18.65.
P/E ratios are highly dependent on capital structure. Leverage (i.e. debt taken on by the company) affects both earnings and share price in a variety of ways, including the leveraging of earnings growth rates, tax effects and impacts on the risk of bankruptcy, and can sometimes dramatically affect the company's results. For example, for two companies with identical operations and taxation regime, and trading at typical P/E ratios, the company with a moderate amount of debt commonly has a lower P/E than the one with no debt, despite having a slightly higher risk profile, slightly more volatile earnings and (if earnings are increasing) a slightly higher earnings growth rate.
At higher levels of leverage (where the risk of bankruptcy forces up debt costs) or if profits decline substantially (driving up the P/E ratio) the indebted firm has a higher P/E ratio than an unleveraged firm.
To try to eliminate these leverage effects and better compare the values of the underlying operating assets, it is often preferable to use multiples based on the enterprise value of a company, such as EV/EBITDA, EV/EBIT or EV/NOPAT.
Since 1900, the average P–E ratio for the S&P 500 index has ranged from 4.78 in Dec 1920 to 44.20 in Dec 1999,. However, except for some brief periods, during 1920–1990 the market P/E ratio was mostly between 10 and 20.
The average P–E of the market varies in relation with, among other factors, expected growth of earnings, expected stability of earnings, expected inflation, and yields of competing investments. For example, when US treasury bonds yield high returns, investors pay less for a given earnings per share and P/E's fall.
Jeremy Siegel has suggested that the average P/E ratio of about 15  (or earnings yield of about 6.6%) arises due to the long term returns for stocks of about 6.8%. In Stocks for the Long Run, (2002 edition) he had argued that with favorable developments like the lower capital gains tax rates and transaction costs, P–E ratio in "low twenties" is sustainable, despite being higher than the historic average.
Set out below are the recent year end values of the S&P 500 index and the associated P/E as reported. For a list of recent contractions (recessions) and expansions see US Business Cycle Expansions and Contractions.
|Date||Index||P/E||EPS growth %||Comment|
|2001-12-31||1148.08||46.50||−30.8||2001 contraction resulting in P–E peak|
|2000-12-31||1320.28||26.41||8.6||Dot-com bubble burst: March 10, 2000|
|1994-12-31||459.27||15.01||18.0||Low P–E due to high recent earnings growth.|
|1990-12-31||330.22||15.47||−6.9||July 1990 – March 1991 contraction.|
|1988-12-31||277.72||11.69||.||Bottom (Black Monday was Oct 19, 1987)|
Note that at the height of the Dot-com bubble P/E had risen to 32. The collapse in earnings caused P–E to rise to 46.50 in 2001. It has declined to a more sustainable region of 17. Its decline in recent years has been due to higher earnings growth.
The P–E ratio of a company is a major focus for many managers. They are usually paid in company stock or options on their company's stock (a form of payment that is supposed to align the interests of management with the interests of other stock holders). The stock price can increase in one of two ways: either through improved earnings or through an improved multiple that the market assigns to those earnings. In turn, the primary drivers for multiples such as the P–E ratio is through higher and more sustained earnings growth rates.
Consequently, managers have strong incentives to boost earnings per share, even in the short term, and/or improve long term growth rates. This can influence business decisions in several ways: