Answer:
Lucky event
Explanation:
In the investments market a true measure of market efficiency is to get a track record of positive outcome from investors over time.
The lucky event problem occurs when an investor makes a profit on investment not because of how efficient a market is or by a logical procedure, but rather by chance.
In the given scenario Keyes put all his money in one stock that doubled in 3 months.
However this was not replicated among other investors who made similar vets on other stocks and lost.
This is an exams of lucky event problem in determining market efficiency.