Which set of earnings is used to work out the P/E of a stock
This is a note from my broker, CMC Markets, who use Morningstar: Morningstar calculate the P/E Ratio using a weighted average of the most recent earnings and the projected earnings for the next year. This may result in a different P/E Ratio to those based solely on past earnings as reported on some sites and other publications. They show the P/E as being 9.93. So obviously past earnings would usually be used but you would need to check with your source which numbers they are using. Also, as BHP's results just came out yesterday it may take a while for the most recent financial details to be updated. "There are two common types of P/E ratio calculations: ""trailing"" and ""forward"" (and then there are various mixes of the two). Trailing P/E ratios are calculated as [current price] / [trailing 12-month EPS]. An alternative is the Forward P/E ratio, which is based on an estimate of earnings in the coming 12 months. The estimate used is usually called ""consensus"" and, to answer your question, is the average estimate of analysts who cover the stock. Any reputable organization will disclose how they calculate their financials. For example, Reuters uses a trailing ratio (indicated by ""TTM"") on their page for BHP. So, the first reason a PE ratio might not jump on an announcement is it might be forward looking and therefore not very sensitive to the realized earnings. The second reason is that if it is a trailing ratio, some of the annual EPS change is known prior to the annual announcement. For example, on 12/31 a company might report a large drop in annual earnings, but if the bulk of that loss was reported in a previous quarterly report, then the trailing EPS would account partially for it prior to the annual announcement.  In this case, I think the first reason is the culprit. The Reuters P/E of nearly 12 is a trailing ratio, so if you see 8 I'd think it must be based on a forward-looking estimate." "@jlowin's answer has a very good discussion of the types of PE ratio so I will just answer a very specific question from within your question: And who makes these estimates? Is it the market commentators or the company saying ""we'd expected to make this much""? Future earnings estimates are made by professional analysts and analytical teams in the market based on a number of factors. If these analysts are within an investment company the investment company will use a frequently updated value of this estimate as the basis for their PE ratio. Some of these numbers for large or liquid firms may essentially be generated every time they want to look at the PE ratio, possibly many times a day. In my experience they take little notice of what the company says they expect to make as those are numbers that the board wants the market to see. Instead analysts use a mixture of economic data and forecasting, surveys of sentiment towards the company and its industry, and various related current events to build up an ongoing model of the company's finances. How sophisticated the model is is dependent upon how big the analytics team is and how much time resource they can devote to the company. For bigger firms with good investor relations teams and high liquidity or small, fast growing firms this can be a huge undertaking as they can see large rewards in putting the extra work in. The  At least one analytics team at a large investment bank that I worked closely with even went as far as sending analysts out onto the streets some days to ""get a feeling for"" some companies' and industries' growth potential. Each analytics team or analyst only seems to make public its estimates a few times a year in spite of their being calculated internally as an ongoing process. The reason why they do this is simple; this analysis is worth a lot to their trading teams, asset managers and paying clients than the PR of releasing the data. Although these projections are ""good at time of release""  their value diminishes as time goes on, particularly if the firm launches new initiatives etc.. This is why weighting analyst forecasts based on this time variable makes for a better average. Most private individual investors use an average or time weighted average (on time since release) of these analyst estimates as the basis for their forward PE."
"If you are looking for numerical metrics I think the following are popular: Price/Earnings (P/E) - You mentioned this very popular one in your question.  There are different P/E ratios - forward (essentially an estimate of future earnings by management), trailing, etc..  I think of the P/E as a quick way to grade a company's income statement (i.e: How much does the stock cost verusus the amount of earnings being generated on a per share basis?). Some caution must be taken when looking at the P/E ratio.  Earnings can be ""massaged"" by the company.  Revenue can be moved between quarters, assets can be depreciated at different rates, residual value of assets can be adjusted, etc..  Knowing this, the P/E ratio alone doesn't help me determine whether or not a stock is cheap. In general, I think an affordable stock is one whose P/E is under 15. Price/Book - I look at the Price/Book as a quick way to grade a company's balance sheet.  The book value of a company is the amount of cash that would be left if everything the company owned was sold and all debts paid (i.e. the company's net worth).  The cash is then divided amoung the outstanding shares and the Price/Book can be computed.  If a company had a price/book under 1.0 then theoretically you could purchase the stock, the company could be liquidated, and you would end up with more money then what you paid for the stock. This ratio attempts to answer: ""How much does the stock cost based on the net worth of the company?"" Again, this ratio can be ""massaged"" by the company.  Asset values have to be estimated based on current market values (think about trying to determine how much a company's building is worth) unless, of course, mark-to-market is suspended.  This involves some estimating.  Again, I don't use this value alone in determing whether or not a stock is cheap. I consider a price/book value under 10 a good number.   Cash - I look at growth in the cash balance of a company as a way to grade a company's cash flow statement.  Is the cash account growing or not? As they say, ""Cash is King"".  This is one measurement that can not be ""massaged"" which is why I like it.  The P/E and Price/Book can be ""tuned"" but in the end the company cannot hide a shrinking cash balance. Return Ratios - Return on Equity is a measure of the amount of earnings being generated for a given amount of equity (ROE = earnings/(assets - liabilities)).  This attempts to measure how effective the company is at generating earnings with a given amount of equity.  There is also Return on Assets which measures earnings returns based on the company's assets. I tend to think an ROE over 15% is a good number. These measurements rely on a company accurately reporting its financial condition.  Remember, in the US companies are allowed to falsify accounting reports if approved by the government so be careful.  There are others who simply don't follow the rules and report whatever numbers they like without penalty. There are many others.  These are just a few of the more popular ones.  There are many other considerations to take into account as other posters have pointed out." "Check your math... ""two stocks, both with a P/E of 2 trading at $40 per share lets say, and one has an EPS of 5 whereas the other has an EPS of 10 is the latter a better purchase?"" If a stock has P/E of 2 and  price of $40 it has an EPS of $20. Not $10. Not $5." No, it is not. If that were the case, you would have no such thing as a growth stock. Dividends and dividend policies can change at any time.  The primary reason for investment in a company is access to a firm's earnings, hence the idea of P/E. Dividends are factored in with capital appreciation, but studies have shown that dividends are actually detrimental to future growth.   They tend to allow easier access to shareholders because of the payouts, reducing the cost of equity. But, if you reduce the growth rate as well, sensitivity tables can demonstrate deterioration or stagnation over time.   Some good examples are GE and Microsoft. "Maria, there are a few questions I think you must consider when considering this problem. Do fundamental or technical strategies provide meaningful information? Are the signals they produce actionable? In my experience, and many quantitative traders will probably say similar things, technical analysis is unlikely to provide anything meaningful. Of course you may find phenomena when looking back on data and a particular indicator, but this is often after the fact. One cannot action-ably trade these observations. On the other hand, it does seem that fundamentals can play a crucial role in the overall (typically long run) dynamics of stock movement. Here are two examples, Technical: suppose we follow stock X and buy every time the price crosses above the 30 day moving average. There is one obvious issue with this strategy - why does this signal have significance? If the method is designed arbitrarily then the answer is that it does not have significance. Moreover, much of the research supports that stocks move close to a geometric brownian motion with jumps. This supports the implication that the system is meaningless - if the probability of up or down is always close to 50/50 then why would an average based on the price be predictive? Fundamental: Suppose we buy stocks with the best P/E ratios (defined by some cutoff). This makes sense from a logical perspective and may have some long run merit. However, there is always a chance that an internal blowup or some macro event creates a large loss.  A blended approach: for sake of balance perhaps we consider fundamentals as a good long-term indication of growth (what quants might call drift). We then restrict ourselves to equities in a particular index - say the S&P500. We compare the growth of these stocks vs. their P/E ratios and possibly do some regression. A natural strategy would be to sell those which have exceeded the expected return given the P/E ratio and buy those which have underperformed. Since all equities we are considering are in the same index, they are most likely somewhat correlated (especially when traded in baskets). If we sell 10 equities that are deemed ""too high"" and buy 10 which are ""too low"" we will be taking a neutral position and betting on convergence of the spread to the market average growth. We have this constructed a hedged position using a fundamental metric (and some helpful statistics). This method can be categorized as a type of index arbitrage and is done (roughly) in a similar fashion. If you dig through some data (yahoo finance is great) over the past 5 years on just the S&P500 I'm sure you'll find plenty of signals (and perhaps profitable if you calibrate with specific numbers). Sorry for the long and rambling style but I wanted to hit a few key points and show a clever methods of using fundamentals." "P/E is a pretty poor way to value the company as it exists today.  The company generates free cash flow yield of 2.5-3.3% which isn't remarkably high, but it's not nearly as bad as the earnings yield of 1.1%.  But let's operate within the P/E ratio for right now.  The company sells $40 billion of ""stuff"" each year with a net margin of 2%.  If they increased prices on every product by 1% (which really wouldn't be _that_ noticeable) their profit would grow 50%.  Thus, P/E drops from 90 to 60.    SGA expenses equal ~20% of their operating costs.  Cut 5% of SGA and you get the same 1% increase in net margin.  This could come from cost-cutting today, or by greater economies of scale in the future while keeping prices the same.    So, yes, it's priced as a growth firm, but it doesn't necessarily need customer growth in the traditional sense to be fairly-valued." I am using the same logic as the two answers above. I got almost the same result ($46.60 instead of $46.59 per share) using the sold fractional share basis. However, the JCI Qualified Dividend (on the 1099-DIV, not the 1099-B) divided by the number of shares spun off yields a basis per share of only $40.97 That compares to $45.349 in answer two above. It seems that we should get the approximately same basis per share using the same arithmetic, and I do not know why we don't. For my tax files, I plan to use the Adient basis equal to the dividend from the 2016 1099-DIV of JCI (the PLC after the merger). My reasoning is that I cannot use an amount for the Adient basis that is greater than the dividend I paid taxes on. [In case this part of the question comes up again, you can get historical quotes at various websites such as https://finance.yahoo.com/quote, which does show $45.51 as the Adient closing price on 10/31/16.] Imagine you have a bank account with $100 in it. You are thinking about selling this bank account, so ask for some bids on what it's worth. You get quotes of around $100. You decide to sell it, but before you do, you take $50 out of it to have in cash. Would you expect the market to still pay $100 for the account? The dividend is effectively the cash being withdrawn. The stock had on account a large amount of cash (which was factored into it's share price), it moved that cash out of it's account (to its shareholders), and as a result the stock instantly becomes priced lower as this cash is no longer part of it, just as it is in the bank account example. "In the long run (how long?) a shares price always reverts to being its proportional amount of the company's residual equity plus the net present value of its expected future cash flows. Or at least that's the theory. In practice PE ratio is used not as a way of measuring what the stock price itself will do but what the fundamental value of holding that share is compared to its price. It is a way of measuring what a company is worth compared to its price and comparing it against other companies to find companies where the underlying value of the company is underrepresented by the price. Comparing PE ratios within the same industry or sector is the most valid use for this (other than comparing previous years of the same company) and the validity of the comparison drops as the structure of the firm you are comparing with gets more different to that of the company. Each industry has its own ""typical"" average PE ratio and these differ wildly between industries so in a great many cases even comparing PE ratios between similar stocks in different industries isn't valid. Any weird pseudo PE ratio that you create for other instruments will be meaningless. In general the best way to compare investments across multiple instruments is by comparing returns. when comparing stocks to other instruments you may want to use the return on stock price or the return on capital employed (ROCE) depending on whether you want to compare the trading performance or the fundamental performance."