By Jonathan Keehner and Michael Flaherty

NEW YORK (Reuters) - Potential investors in the Blackstone Group are likely to be left scratching their heads when they try to put their fingers on the company's bottom line.

In a filing issued on Monday for its upcoming public offering, Blackstone makes it clear that net earnings won't be one of its key metrics. Instead, bearing in mind the company's warning that it expects significant net losses for years, investors should get familiar with less common measurements of performance, such as "pro forma economic net income."

With many other private-equity firms and hedge funds expected to pursue a path similar to Blackstone's, Monday's filing shows that investors are likely to be flummoxed when it comes to understanding the earnings of these private partnerships that go public.

Management at Blackstone looks to "economic net income" -- or net income excluding the impact of income taxes, noncash charges related to vesting of equity-based compensation and amortization of intangible assets -- as a "key performance measure," according to the filing.

Blackstone said economic net income should not be considered in isolation or as an alternative to income before taxes in accordance with GAAP -- Generally Accepted Accounting Principles.

While net income is considered a company's bottom-line profit, a quick search through databases finds the term "economic net income" used by only one other major publicly traded company in the past year, Southwest Airlines Co

(LUV.N: Quote, Profile, Research).

It isn't the only term used by Blackstone that will have investors reaching for financial dictionaries. They should also get used to terms like "finite-lived intangible assets."