Before private equity titan Carlyle Group joins Blackstone Group LP, Kohlberg Kravis Roberts & Co. LP, Apollo Global Management LLC and Fortress Investment Group LLC in the ranks of publicly traded leveraged buyout firms, investors will tackle the question of what it's worth.

That's a tough thing to answer, even for PE firms whose shares sport explicit prices. Carlyle filed to go public on Sept. 6 and is expected to list early next year. Co-founder David Rubenstein had talked of the likelihood of his shop and others selling stakes to the public since 2004.

One axiomatic truth is that private equity firms today don't deserve the lofty valuation of 22 times earnings at which Blackstone's shares blasted off when Steve Schwarzman took it public in 2007, only to plummet along with the economy soon after. Investors have wised up to the sector's topsy-turvy cyclicality.

But cyclicality isn't the only difficulty the industry presents. A bigger puzzle is the squishy nature of earnings figures, along with idiosyncratic accounting that takes a while to get the hang of.

Start with the accounting. PE firms are unlike manufacturers or retailers, whose profits materialize in short order. Much of a buyout firm's gains are bottled up in intermittently traded, hard-to-evaluate assets and can take years to realize. Because of that, and even more because of quirks of business structure too arcane to address here, widely used methods following generally accepted accounting principles muddy the performance picture of PE firms.

While the public PE firms dutifully report GAAP numbers, they've hatched a new array of financial metrics to give more clarity.

While the metrics do that, they also come with their own set of enigmas. Take the way the firms tally revenue and earnings, which most of these firms massage into a metric called economic net income, or ENI. (Fortress is an exception.) With both, future performance fees the firms stand to collect on their unsold investments (typically 20% of cash investment gains) are estimated and factored in.

The problem with this is threefold: First, a big chunk of revenue and ENI is phantom. Carlyle, for one, collected just 52% of the revenue it registered over the past year, and Blackstone 53%. Second, it's based on asset valuation "marks": judgment calls, albeit carefully produced and thoroughly vetted, of what the unsold holdings are worth. Third, the revenue and ENI figures reflect what those holdings are worth today. By the time they're actually unloaded, the market climate and asset valuations are liable to have changed.

Then there is this oddity: The performance fees layered into revenue and ENI are relative metrics. Specifically, they mirror changes in unrealized fees the firms stand to collect, based on quarter-to-quarter or year-to-year changes in the asset marks.

As a result, they can give a distorted image of investment performance. Say the overall value of Firm X's investments climbs from 1.1 to 1.5 times original cost in a given year, while that of Firm Y, equal in size to Firm A, rises from 2.3 to 2.35 times cost. In that case, Firm X's ENI may be a lot higher, even though Firm B's investments are performing far better.

Beyond that, while the firm's definitions of revenue and ENI are broadly consistent, the figures do not always match up. Perhaps the most glaring discrepancy: Henry Kravis' KKR excludes from stated compensation costs the payouts that it makes to senior partners from a discretionary pool. In 2010 that omission boosted KKR's ENI by about $200 million.

Blackstone adds a touch of polish to the overall annual internal rate of return it reports in private equity. While Carlyle and KKR figure their IRRs based on every dollar they've deployed over time, Blackstone removes unrealized deals (read: its worst recent investments) from its composite computation.

Keeping those elusive metrics in, we come back to the question of Carlyle's market value. Would-be investors in Carlyle's initial public offering and bankers will inspect the diversity and reliability of Carlyle's income streams, then evaluate them against the public "comps."

Public-market investors prize diversity in PE firms because it can blunt the impact of cyclical slumps. Blackstone, the most multidimensional of the group, trades at about 6 times trailing pretax ENI. Though Carlyle, Apollo and KKR have made strides to catch up, they lag well behind Blackstone on that score. What's more, Carlyle's splashy recent acquisition of fund-of-funds manager AlpInvest Partners NV is not expected to elevate ENI all that much.

Were the IPO to happen today, Carlyle might command a value of 2.5 to 3.5 times ENI. That works out to a market capitalization for Carlyle of $4 billion to $6 billion.

So the numbers suggest. For whatever they're worth.

David Carey covers private equity for The Deal magazine.