Quote from HotTip:
I looked at the idea some time ago, and did a comprehensive data analysis over almost a thousand ERs. My conclusion was that it was possible to extract a profit if you monitored the price action prior to the ER, because more often than not a stock that significantly trends in the day(s) prior to the ER will reverse immediately after the ER, regardless of the news, as all the traders attempted to capture a profit. Unfortunately I abandoned the idea because there were too many 25%+ hits. The extremely high volatility meant that I'd have to scale down my bets considerably, yielding an overall profit that was less than desirable. What I trade now is much less volatile and also much more consistently profitable.
I agree with Truthteller though. Just by analyzing the fundamentals it is impossible to predict how a stock will do post-ER, as all of the fundamentals (even whisper numbers) are already captured in the stock price. I likewise attempted to predict post-ER price movement based on guidance, whisper numbers, ratings, etc., and found no success. Price action was the key.