Private information here refers to whatever insights you bring with your research. The point is that if you gain some private information (by being a quant, ana

Private information here refers to whatever insights you bring with your research. The point is that if you gain some private information (by being a quant, analyst, etc.) you would immediately trade on it and the price would then start reflecting your views. Also EMH, and that's the point Fama uses a lot to battle critics, does not put a time frame on when the price adjustment happens.