In Efficient market hypothesis, the third form is strong form of EMH, which suggests privately available information is also reflected on stock price.
1. what is empirical revelance of strong form of EMH?
2. how can be hypothesize the private information is reflected on stock price, unless it is not available to all the investors in the market.
3. if such private information is accessable to all the investors, it no longer be private information. it will be public information only?
1. you can try to reject strong form of EMH by various tests. For example, if stock price increases before companies announce surprisingly high earnings, and decreases before companies announce surprisingly low earnings, it would be an evidence against the strong form of EMH.
2. This is not a problem. Market aggregate information. People trade on their information. If you are the only person who poses some information, and you go and trade based upon this information, then this information will be reflected in market price.
3. Indeed, if information would be available to everybody, it would not be a private information.
The argument of strong form is not that private information is publicly known, as then it would not be private information. The point is that the value of that private information is incorporated into the share price even though the cause (private information) remains hidden from public view. This is because the holders of private information have impacted the share price.
I assume you have read Fama. If not then here is a copy for your academic research purposes.
With regard to Milan's reference to Berkshire Hathaway, it is well known that the Chairmen and CEO of Berkshire reject all forms of Efficient Market Hypothesis. So you may wish to compare and contrast with their contrary viewpoints.
The literature review in the following doctoral thesis (chapter 2) may also help your quest:
https://www.researchgate.net/publication/267869578_An_analytical_tool_to_aid_the_reflective_selection_of_equity_investments