* Trading costs average around 1.5 percent for funds

* Specialists seek firmer measures to count such costs

By Ross Kerber

BOSTON June 3 (Reuters) - Trading costs for mutual funds -- largely hidden from investors -- average around 1.5 percent a year, higher than many disclosed expenses, two new studies found.

"If you can show the actual trading costs, the hidden part of the iceberg is finally revealed," said Arijit Dutta, associate director of research at Morningstar Inc, who authored one of the studies.

Separately, University of California Professor Roger Edelen and two co-authors found that trading costs are comparable to funds' expense ratios and often exceed them. "We think our study really nails down that these costs do matter more than expense ratios," Edelen said.

The researchers presented their findings at an investment conference sponsored by Chicago-based research firm Morningstar last week. The Morningstar study is at a preliminary stage, while Edelen's is being reviewed by peers before it is published.

Trading costs, which mutual funds incur for their investors when they trade stocks, have been one of the fund industry's most vexing subjects because there are no agreed-upon standards to account for them. This contrasts with traditional fund expenses such as the salaries paid to fund managers or accountants.

In Washington the subject is controversial as moves gain steam to require more fee disclosure around company 401(k) savings plans, which invest about half their assets in mutual funds.