By Jacob Bunge 

DuPont Co. on Tuesday will detail fourth-quarter earnings and project profits for the year ahead--but how much weight investors should put on the forecast depends these days on whom you ask.

The Delaware-based chemical company is embroiled in a battle with activist investment firm Trian Fund Management LP, which has blasted DuPont's ability to forecast its own performance.

The firm, led by Nelson Peltz, argues DuPont's conglomerate structure mires its businesses in excess costs and complexity, making it tough for the company to deliver reliable earnings guidance for investors.

DuPont has fired back, pointing to performance the company says exceeded three-year targets set in 2009 and longer-term goals laid out in 2010. It also trumpets that its shares beat the broader stock market over the past five years.

The dispute will put added scrutiny on DuPont's fourth-quarter results, which will reveal whether the company delivered a full-year profit within its expected range of $4.00 to $4.10 a share.

Stock analysts regard earnings forecasts as an imprecise science, often buffeted by world events, weather or other calamities. But they are almost always better than no forecasts at all, analysts say.

A few companies enjoy clockwork-like, recurring business that is easy to track, and are masters of meeting and exceeding earnings targets. Basic chemical companies with highly volatile performance occupy the other end of the spectrum, according to John Roberts, analyst at UBS AG. "DuPont's somewhere in the middle," he said.

Trian argues DuPont's track record is unusually spotty. The fund, which this month launched a proxy fight at DuPont by nominating four candidates to the company's board, argues that DuPont already faltered on its latest guidance last June, when it scaled back 2014 earnings expectations from its earlier estimate of $4.20 to $4.45.

Trian also contends that DuPont has lowered or missed its guidance for the past three years--though an examination of those years shows the argument isn't cut and dried.

-- 2012: DuPont in December 2011 said it expected to earn in 2012 $4.20 to $4.40 a share, excluding "significant items," an estimate executives reaffirmed in January and April 2012 before telling investors in July that results would likely fall at the lower end of the range. In October 2012, DuPont lowered its range to $3.25 to $3.30 a share. For the full year, DuPont reported earnings of $3.33 after stripping out one-time items.

-- 2013: DuPont in January 2013 projected full-year operating earnings of $3.85 to $4.05 a share, reaffirming that range in April before zeroing in on "about $3.85" a share in July. DuPont stuck with that figure in October and after its fourth quarter reported $3.88 a share in operating earnings. Trian accused DuPont of "manufacturing" 27 cents worth of profit in 2013, by counting some seed income that otherwise would have registered in the following quarter, and including some nonrecurring investment gains. DuPont says it met the guidance through "disciplined execution, higher volumes, innovative products and productivity gains," some of which let the company deliver corn seed to customers earlier than usual, according to a spokesman.

-- 2014: DuPont in January 2014 projected $4.20 to $4.45 a share in full-year operating earnings, which the company reaffirmed in April but scaled back in June to between $4.00 and $4.10 a share, owing to slower corn seed sales and other factors. Trian made public its critique of DuPont's management in September.

Some analysts say forecasting DuPont's performance is a tricky task, given its businesses span corn fields, chemical plants, auto factories and food company kitchens around the world.

"The more complex your business, the tougher it is to issue guidance," said Don Carson, analyst with Susquehanna Financial Group.

Write to Jacob Bunge at jacob.bunge@wsj.com

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