WASHINGTON — The National Restaurant Association’s Restaurant Performance Index fell 0.9% to 101 as restaurant operators faced challenging economic conditions.

“The September decline in the R.P.I. was due in large part to a dampened outlook among restaurant operators for business conditions in the months ahead,” said Hudson Riehle, senior vice-president of the Research and Knowledge Group for the N.R.A. “Restaurant operators remain unimpressed about the direction of the economy, with elevated food costs remaining at the top of their list of challenges.”

The R.P.I. features two components: the current situation index (C.S.I.) and the expectations index (E.I.). The September E.I. fell 1.1% from August. Restaurant operators remain generally positive about sales gains in the coming months. Forty per cent of restaurant operators said they expect to have higher sales in six months (compared to the same period in the previous year), down slightly from 45% who reported similarly last month. Eight per cent of restaurant operators expect their sales volume in six months to be lower than it was during the same period in the previous year, up from 5% in August.

While restaurant operators are relatively optimistic about sales growth, their outlook for the economy remains mixed. Twenty per cent of restaurant operators said they expect economic conditions to improve in six months, while 19% expect the economy to worsen. The remaining 61% expect economic conditions in six months to be about the same as they are now.

The C.S.I. fell 0.8% in September to 101. A majority of restaurant operators reported higher same-store sales for the seventh consecutive month. Sixty-three per cent reported a same-store sales gain between September 2013 and September 2014, essentially unchanged from the 62% who reported higher sales in August.

Only 23% of operators reported a same-store sales decline in September, up slightly from 21% in August.

In contrast, restaurant operators reported somewhat softer customer traffic results in September. Forty per cent of restaurant operators reported an increase in customer traffic between September 2013 and September 2014, while 33% reported lower customer traffic. In August, 45% reported higher customer traffic, while 31% reported a decline.