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.

Operators are pessimistic about the direction of the overall economy as only 17% of operators said they expect economic conditions to improve in six months, down from 21% last month. Also, 29% of operators said they expect economic condition to worse in the next six months.