A drop in corporate bond yields probably hurt the typical large U.S. defined benefit pension plan in August.

The “funded ratio” at a typical plan fell to 66.9% at the end of the month, from 67.6% at the end of July, according to consultants in the Stamford, Conn., office of Towers Perrin Forster & Crosby Inc.

Towers Perrin bases pension status figures partly on the performance of a hypothetical benchmark pension plan. The firm also tracks the actual pension results reported by 300 large U.S. companies.

The funded ratio has dropped 23.3%, from 87.2, since August 2008, Towers Perrin reports.