Investors dread uncertainty.

But much remains up in the air as


(GOOG) - Get Report

gears up to announce fourth-quarter earnings after the bell on Thursday.

That may be why the stock has recently performed so poorly -- Google shares are off about 20% since the start of the year -- and the hesitancy will set the tone for Thursday's announcement.

Take the company's core search advertising business. According to data by comScore, Google's search traffic growth slowed to 8% in the fourth quarter from 11% the quarter before and caused the hit to Google shares, American Technology Research analyst Rob Sanderson wrote in a research note Tuesday.

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But it's unclear how useful traffic data is in predicting what investors really care about: financial results. Google gets paid when users click on text ads. And while search traffic plays an important role in determining revenue, so do a host of other factors. These include the amount advertisers are bidding to place ads and the number of ads actually being clicked.

Indeed, in last year's fourth quarter, comScore data suggested that search traffic at Google grew 10% from the prior quarter, but net revenue was up 20%, Sanderson wrote. "In our experience, we have not found comScore data to be useful in projecting revenue for Google."

Other analysts also believe investors may be reading too much into the traffic numbers.

"We believe the slower y/y growth spooked many investors as an indication that Google's core search dominance is waning," Thomas Weisel analyst Christa Quarles wrote in a research note Monday. "We think it is unwise to make investment decisions on this relatively new data set from comScore given it has still not proven its predictive value." Thomas Weisel makes a market in Google shares.

Still, the data add another element of uncertainty for a company that has yet to be tested by an economic downturn, an event that seems increasingly likely.

How Google would fare in a recession remains a topic of considerable debate. A slowdown could actually boost Google's comparatively cheap and effective form of advertising. But a severe decline would ultimately hit the company, too.

"We continue to believe that marketers will shift budgets to high ROI and direct response online marketing in the event of a recession, which should play well into Google's positioning," Quarles wrote.

"Ultimately, however, if consumers can't afford a new home and don't have the money for digital cameras, then that reduction in end demand would manifest through a contraction in commercialziable queries leaving no place for those advertising budgets to go."

But beyond search, Google must now define a concrete plan to move into new markets like display advertising, mobile, and video advertising.

Google scored a key win this quarter when the Federal Trade Commission cleared its purchase of online ad serving company DoubleClick. But the deal is still pending approval from European regulators. Investors will be listening closely for updates on those proceedings.

Regarding its mobile foray, Google made a big push in November by announcing Android, a software platform for mobile devices. The company has frequently pointed to 2008 as the year that mobile advertising finally takes off. Investors will expect a game plan for real results soon.

The company also introduced ads on its YouTube video sharing service in 2007. Google now has had plenty of time to experiment with the new ad format and could launch the ads on a larger scale.

At any rate, investors will likely appreciate anything resembling certainty from Google on Thursday.

Analysts surveyed by Thomson Financial/First Call expect the company to earn $4.44 a share on revenue of $3.45 billion.