WASHINGTON — The National Restaurant Association’s Restaurant Performance Index declined to 99.7 in December, down 0.2% from November. It also represented the third month when the R.P.I. has been below 100.

“Although restaurant operators reported softer same-store sales and customer traffic levels in December, they are cautiously optimistic about sales growth in the months ahead,” said Hudson Riehle, senior vice-president of the Research and Knowledge group for the N.R.A. “However, operators remain decidedly pessimistic about the overall economy with only 17% saying they expect business conditions to improve in the next six months.”

Restaurant operators reported positive net same-store sales for the 19th consecutive month, but December’s results were softer than November’s. Forty-two per cent of restaurant operators reported a same-store sale gain between December 2011 and December 2012, which was down from 55% of reported positive sales in November. Also, 38% of operators reported lower same-store sales in December, up from 30% in November. There was also a net decline in customer traffic levels in December with 31% of operators reporting higher customer traffic levels, down from 43% who reported positive traffic in November. Also, 48% of operators had lower customer traffic levels in December, up from 35% in November.

Thirty-seven per cent of operators expect to have higher sales in six months, and 16% of operators expect their sales volume in six months to be lower than it was during the same period of the previous year.