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Which set of earnings is used to work out the P/E of a stock
@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.