Strong form of Market Efficiency
This is a very strong premise, hence the strong form, and it suggests that all information is fully reflected in the prices, and going a step further than you might intuitively, it says it reflects information whether it is publicly available or not!
The other strong outcome of this is it means that for any market that truly operates in this way, no investor can make a better than average return in the long run through anything other than pure luck, and this is simply because no matter what research they do everything that could be taken into account has been taken into account, so knowing some privileged piece of data that might impact prices is no good because it's already been factored into the price levels.
There are quite interesting tensions between those that work on the markets... most researchers believe in EMH and the semi-strong theory for the market, but of course the actual analysts disagree - after all they don't want to lose their jobs do they!
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