How to calculate PE ratios for indices such as DJIA?
The official source for the Dow Jones P/E is Dow Jones.  Unfortunately, the P/E is behind a pay-wall and not included in the free registration. The easiest (but only approximate) solution is to track against an equivalent ETF.  Here's  a list of popular indexes with an equivalent ETF. Source You could look up the P/E of an equivalent ETF, or break the ETF into components and look those up. Each index has its own methodology, usually weighted by market cap.  See here: http://www.amex.com/etf/prodInf/EtAllhold.jsp?Product_Symbol=DIA One thing to keep in mind when calculating P/E on an index is that the E (earnings) can be very close to zero.  For example, if you had a stock trading at $100 and the earnings per share was $.01, this would result in a P/E of 10,000, which would dominate the P/E you calculate for the index.  Of course negative earnings also skew results. One way to get around this would be to calculate the average price of the index and the earnings per share of the index separately, and then divide the average price of the index by the average earnings per share of the index. Different sources calculate these numbers in different ways.  Some throw out negative P/Es (or earnings per share) and some don't.  Some calculate the price and earnings per share separate and some don't, etc... You'll need to understand how they are calculating the number in order to compare it to PEs of individual companies.
It is not so useful because you are applying it to large capital. Think about Theory of Investment Value. It says that you must find undervalued stocks with whatever ratios and metrics. Now think about the reality of a company. For example, if you are waiting KO (The Coca-Cola Company) to be undervalued for buying it, it might be a bad idea because KO is already an international well known company and KO sells its product almost everywhere...so there are not too many opportunities for growth. Even if KO ratios and metrics says it's a good time to buy because it's undervalued, people might not invest on it because KO doesn't have the same potential to grow as 10 years ago. The best chance to grow is demographics. You are better off either buying ETFs monthly for many years (10 minimum) OR find small-cap and mid-cap companies that have the potential to grow plus their ratios indicate they might be undervalued. If you want your investment to work remember this: stock price growth is nothing more than You might ask yourself. What is your investment profile? Agressive? Speculative? Income? Dividends? Capital preservation? If you want something not too risky: ETFs. And not waste too much time. If you want to get more returns, you have to take more risks: find small-cap and mid-companies that are worth. I hope I helped you! This is only a rule of thumb. Peter Lynch popularized it; the ratio PE/growth is often called the Lynch Ratio. At best it's a very rough guideline. I could fill up this page with other caveats. I'm not saying that it's wrong, only that it's grossly incomplete. For a 10 second eyeball valuation of growth stocks, it's fine. But that's the extent of its usefulness. Usually their PE ratio will just be listed as 0 or blank.  Though I've always wondered why they don't just list the negative PE as from a straight math standpoint it makes sense. PE while it can be a useful barometer for a company, but certainly does not tell you everything.  A company could have negative earnings for a lot of reasons, some good and some bad.  The company could just be a bad company and could be losing money hand over fist, or the company could have had a one time occurrence such as a big acquisition or some other event that just affected this years earnings, or they could be an awesome high growth company that is heavily investing for their future and forgoing locking in profits now for much bigger profits in the future. Generally IPO company's fall into that last category as they are going public usually because they want an influx of cash that they are going to use to grow the company much more rapidly.  So they are likely already taking all incoming $$ and taking on debt to grow the company and have exceeded all of those options and that's when they turn to the stock market for the additional influx of cash, so it is very common for these companies not to have earnings.   Now you just have to decide if that company is investing that money wisely and will in the future translate to actual earnings. Traditionally, the Dow Jones Industrial Average (DJIA) was only comprised of stocks that were traded on the New York Stock exchange.  Neither Apple (AAPL) nor Google (GOOG) are traded on the New York Stock Exchange but instead are traded on NASDAQ.  All NASDAQ tickers are four characters long and all NYSE tickers are only three or less characters long (e.g. IBM or T (AT&T)). However in 1999, MSFT became the first NASDAQ stock to be included in the DJIA.  Given that AAPL now has the largest market capitalization of any company in U.S. history, I think it is likely if they retain that position, that they would eventually be let into the DOW club too, perhaps, ironically, even supplanting Microsoft. Efficient Frontier has an article from years ago about the small-cap and value premiums out there that would be worth noting here using the Fama and French data. Eugene Fama and Kenneth French (F/F) have shown that one can explain   almost all of the returns of equity portfolios based on only three   factors: market exposure, market capitalization (size), and   price-to-book (value). Wikipedia link to the factor model which was the result of the F/F research. PE can be misleading when theres a good risk the company simply goes out of business in a few years. For this reason some people use PEG, which incorporates growth into the equation. "If you are a ""small"" investor (namely, not an accredited investor), then the transaction costs (commissions) for purchasing the stocks while attempting to duplicate DJIA will defeat any benefit. My personal preference is to purchase mutual funds rather than ETFs." The S&P 500 is a market index.  The P/E data you're finding for the S&P 500 is data based on the constituent list of that market index and isn't necessarily the P/E ratio of a given fund, even one that aims to track the performance of the S&P 500.    I'm sure similar metrics exist for other market indexes, but unless Vanguard is publishing it's specific holdings in it's target date funds there's no market index to look at.