Well, the weak form of the efficient market hypothesis states

that prices reflect all marketing data.

Now what is marketing data?

Well, it's things like past prices, trading volume, or short interests, right?

So basically, this version of the hypothesis implies that you

can't predict future prices by looking at past prices or past volume, right?

So what does this mean?

Well, this means for example, that trend analysis is completely useless, right?

If markets are weak form efficient, you can't use any past trading data,

price or volume data, that is publicly available and virtually costless

to obtain to predict future stock prices or future performance, right?

And if such data were able to predict future prices,

then all investors would have already exploited those signals, and

therefore that information would already be reflected in the prices, okay?

So that's the weak form of the efficient market hypothesis,

where the available information just is the market information price data, right?

Now, what about the second form?

Well, the second form,

the semi-strong form of the efficient market hypothesis states that all

publicly available information should already be reflected in prices.

Now, what do we mean by publicly available information?

Well now, in addition to the past market trading data that I mentioned, think

of all the information that is available about a company that is public, right?

Its accounting data, its financial data, fundamental data, its patents,

product line, earnings forecast.

Anything you can find out about a company in the public domain.

So if markets are semi-strong efficient, one should not be able to predict prices

based on any such public data because that would already be reflect in prices, okay?

All right, so finally, the strong-form of the efficient marketing hypothesis.

All right, that one states that prices reflect all information that is relevant.

Public and including all information that is available only to company insiders.

Now clearly, right?

You can see that this is a very tall order, right?

And this version of the hypothesis is very extreme.

Now, undoubtedly, company insiders are likely to have

valuable information that is not public and

that will allow them to profit from their information, right?

And in fact, the ACC in the US tries to prevent exactly this kind of situation

where insiders can exploit their private information to profit, right?