Companies that surpass Wall Street s earnings forecasts tend to outperform the broad stock market. Surprisingly, they tend to produce handsome returns gradually over many months, rather than all at once on the day of the report. Stock market researchers have observed that phenomenon for decades and have dubbed it post earnings announcement drift.

No one is sure exactly why the drift occurs, but many theories rely on investor psychology. For example, one explanation holds that investors become so anchored in their beliefs that they fail to quickly incorporate new evidence, including signs that a company is performing better than expected.

In a 2006 study published in the Journal of Finance, professors Narasimhan Jegadeesh of Emory University and Joshua Livnat of New York University demonstrate that investors can sharply increase the predictive power of earnings surprises by looking for sales surprises, too. That makes sense; some companies beat earnings forecasts through short-term measures like cost-cutting, but companies that outperform by simply bringing in more money than expected have a better chance of sustaining their success. In the study, the top 20% of earnings outperformers went on to beat the broad market s returns by three percentage points over the following six months, and the top 20% of sales outperformers beat the market by a little less. A cross-sample of the two companies that outperformed on sales and earnings beat the market s returns by more than five percentage points over the following six months.

Roughly two-thirds of S&P 500 companies have so far reported quarterly financial results for the index s fourth quarter of 2009. Only one-quarter of these have managed to increase their sales and operating profits from a year ago while topping Wall Street s forecasts for both measures. Three are listed below.

Boeing

In mid-December, Boeing not only produced its next-generation jumbo jet for photographers but also put it in the air (and brought it down, safely). The 787 s maiden flight was about two years behind schedule, but a welcome sight nonetheless for airlines that have pre-ordered the fuel-efficient plane. Boeing s sales and earnings success last quarter was driven by decent demand for its other models, lucrative defense contracts and a reduced headcount. The company expects to deliver only a few 787s by the end of this year with production peaking by 2013. Analysts reckon earnings will peak then, too. The last time Boeing s profits peaked, they did so at a yearly rate of about $6.25 a share. The company is earning now at a pace closer to $4 a share. In other words, a stock price that works out to just over 15 times earnings today might be less than 10 times earnings a few years from now. The stock comes with a 2.8% dividend yield.

Forest Laboratories

Forest Laboratories trades at less than nine times earnings for a reason. In about two years, the company is expected to lose $2.3 billion of its $4.2 billion in yearly sales when its patent for Lexapro, a pill for depression, expires. Management is busy shopping for mid- and late-stage developmental medicines to buy and usher to market. The company expects six drug approvals between now and the end of 2012. Forest s strong financial results in its most recent quarter are a reminder of how profitable the company remains for now. It generates yearly cash flow equal to nearly 12% of its stock market value, and it sits on a cash stockpile equal to more than 35% of its stock market value.

Starbucks

Starbucks reported 4% sales growth at longstanding stores in its most recent quarter, versus a sales decline in the preceding quarter. In the U.S., customer traffic was flat but sales per customer rose. Overseas, just the opposite: the sales increase was owed to increased customer traffic. In the U.S., the company s Via instant coffee is selling well; reduced waste in stores has improved margins; and, according to analysts, customer satisfaction scores are up, thanks in part to menu changes. The company has nearly $1 billion in net cash, and should add another $1 billion this year. What more could investors/customers want? As a dividend zealot and coffee snob, I vote for some yield and a less-roasty brew.