Is the market cheap, as investors like Bruce Berkowitz and David Dreman have said recently? Or is it still overpriced, as others, like Jim Rogers and Bill Gross, have maintained? The answer may depend on which set of corporate earnings you look at.

That’s the issue The Wall Street Journal’s Mark Gongloff examines today. Gongloff discusses the difference between operating earnings (which exclude supposed one-time expenses like writedowns to give a better idea of the profitability of a company’s underlying business) and reported earnings (which include those “one-time” expenses). The reported earnings are often less than operating earnings during tough times, Gongloff notes, as was the case in 2000-02. But after a downturn or recession lifts and unusual losses fade away, operating earnings and reported earnings should be pretty close to each other.

The problem, Gongloff writes, is that that didn’t happen as the economy turned around from 2002-2007. Companies kept posting operating earnings that were higher than the earnings they actually booked for the quarter or year, a sign that those “one-time” recession/downturn-driven losses or expenses weren’t really “one-time”. That likely inflated investors’ assessments of the real earnings power of a firm’s business.

Today, the S&P 500 is selling for less than 14 times earnings using the past four quarters’ operating earnings, but almost 20 times reported earnings for the period, Gongloff notes.

Jeremy Grantham of the investment firm GMO tells Gongloff that “the quality of earnings isn’t what it used to be,” adding that one factor involves companies failing to depreciate the value of technology quickly enough on their books. Grantham chops 13 percent off of operating earnings when evaluating a company, the average gap between operating earnings and reported earnings for the past 15 years, Gongloff reports.