WASHINGTON — In September, restaurant industry performance remained soft. The National Restaurant Association relays its comprehensive index of restaurant activity declined for the second consecutive month.

The association’s Restaurant Performance Index (R.P.I.), which is a monthly composite index that tracks the health of and outlook for the U.S. restaurant industry, was 97.5 in September, down 0.4% from August and its 23rd consecutive month below 100.

Although there were signs in recent months the short-term outlook may be improving, the latest figures indicate the restaurant industry’s recovery has yet to fully gain traction, said Hudson Riehle, N.R.A. senior vice-president of the Research and Knowledge Group. "Restaurant operators continued to report declines in same-store sales and customer traffic in September, and their outlook for sales growth in the months ahead remains mixed."

The R.P.I. is based on the responses to the association’s Restaurant Industry Tracking Survey, which is fielded monthly among restaurant operators nationwide on a variety of indicators including sales, traffic, labor and capital expenditures. The R.P.I. consists of the Current Situation Index and the Expectations Index.

Although restaurant operators continued to report negative same-store sales in September, the performance was an improvement over the record-low reading posted in August. Twenty-two percent of restaurant operators reported a same-store sales gain between September 2008 and September 2009, up from just 17% who reported a sales gain in August. Sixty-five percent of operators reported a same-store sales decline in September, down slightly from 68 who reported negative sales in August.

Customer traffic results also improved slightly in September, despite remaining negative for the 25th consecutive month.