[lutusp]

> The strongest form claims all data, but no one believes that (insider trading alone debunks it).

Insider trading doesn't "debunk" the EMH, because the EMH is predicated on fair dealing. There's a lot of distorted ideas about the EMH, for example that a market crash, or cheating, disproves it. These events don't disprove the hypothesis.

[gnaritas]

> These events don't disprove the hypothesis.

Then the hypothesis is useless and false. You can't claim markets are efficient and then exclude all evidence to the contrary.

> because the EMH is predicated on fair dealing

Yes, which makes EMH false in the real world. A hypothesis that's only true in an ideal world isn't useful nor is it meaningful in the real world.

[Guvante]

> A hypothesis that's only true in an ideal world isn't useful nor is it meaningful in the real world.

You are not treating EMH like you should, think of it as a guideline not a hard and always true rule of life.

For instance lets say Telsa is trading for $100 and you think it is worth $50, there are two reasons for this, either:

* The market is not efficient and you should take advantage. * The market is efficient and you aren't considering all of the variables.

In almost every conceivable case it is the latter. The answer for Tesla is probably optimism for the technology when optimism often drives prices above the otherwise expected value.

EMH makes you consider the market as the baseline, which greatly helps steer you towards what is going on.