PLANO, Texas (AP) _ Perot Systems Corp. lowered its revenue and earnings figures for the first two quarters of this year after its auditor revised its interpretation of accounting rules.

The company, which runs computer systems for customers, said Thursday that the change would cut earnings by a penny per share for each quarter.

The company said in a filing with the Securities and Exchange Commission that it earned $4.9 million, or 4 cents per share, in the second quarter and lost $23.1 million, or 20 cents per share, for the first six months of the year.

Sales were lowered $2.5 million for the first six months of the year, to $696 million, the company said.

The revisions reflected changing accounting rules that make it harder to count revenue from a contract before work is completed.

Perot Systems said the change in accounting principles would not affect its projections for the third quarter, which call for earning 10 cents per share to 13 cents per share on revenue of $360 million to $375 million. Analysts surveyed by Thomson First Call were expecting earnings of 12 cents per share in the third quarter.

The change was announced after markets closed Thursday. In midday Friday trading on the New York Stock Exchange, Perot Systems shares were down 15 cents to $10.20.