There is plenty of empirical evidence that earnings surprises on Wall Street have a material effect upon the stock price of the stock in question, which has been documented by many institutions as well as brokerage firms. That is to say, when a given company exceeds or misses their quarterly earnings consensus number and/or their quarterly sales goals, there will be a material impact in the price of that stock that will be reflected in their updated stock price. Not too surprisingly, it is when earnings and/or sales disappoint, that one will see the biggest percentage impact on the stock price, because nobody wants to be that stupid sucker holding the wrong stock; so as earnings surprises go, one would prefer pre-knowledge of negative news because it is far more profitable.
Obviously, those that are in the highest levels of management, finance, and authority within a given company, have a very clear idea as to whether quarterly earnings are going to be pleasantly received by Wall Street or not. Because the stock exchanges wants to at least appear to be fair to the common investor, "quiet" periods have been instituted for publically traded companies which in theory, helps to level the investing field, by essentially eliminating any direct information being disseminated by said company to security analysts and the like in regards to definitive information on earnings.
Of course, when it comes to money and especially to the making of money that directly pertains to you, this so-called "quiet" period is violated either directly or indirectly time and time again. There have been numerous cases of insider trading to which prison sentences and/or monetary fines have been imposed, and no doubt, there will continue to be criminal cases like this far into the foreseeable future, mainly because getting advanced information about specific companies is highly correlated to impressive overall stock trading success and also monetary rewards for those disseminating such.
The way that Wall Street works is that advanced information is king, and those that are privy to this information have an inherent and unfair advantage over those that do not. For instance, in today's hi-technology world, to which stock buys and sells are executed within fractions of a second, just having knowledge that is a minute or two ahead of the actual earnings surprise, will, in a short period of time, make you a very successful and profitable trader. To take this to the next level, recognize that Wall Street is filled with ambitious, competitive, cutthroat, and motivated people that are hungry to make money, and further to the point that most highly visible public companies do not want to get on the bad side of brokerages, or stock analysts, or media, and there is a basic symbiotic reason why at a minimum some sort of implied notice of privileged information is leaked one to another.
For the common investor, you are always at an inherent and material disadvantage when it comes to your equity investments, especially over the short term and consequently there isn't much that you can do about that, because by the time the news gets to you, the easy money has already been made. At publically traded companies, earnings surprises are a fact of life, and this information is ready-made for insiders and their associates to profit from, and it's no real surprise that it's done, because many people desire that little "extra" just because.