WASHINGTON — The National Restaurant Association’s Restaurant Performance Index declined 0.8% during the month of January to 100.2 compared with the month of December. The N.R.A. cited difficult weather conditions as the primary reason for the decline.

“The R.P.I.’s January decline was due in large to part to dampened sales and traffic levels as a result of extreme weather in some parts of the country,” said Hudson Riehle, senior vice-president of the research and knowledge group for the N.R.A. “Although restaurant operators reported softer same-store sales and customer traffic results in January, their outlook for sales growth and the economy remained optimistic.

“Overall, the economic fundamentals of the restaurant industry remain positive, which will likely lead to stronger sales and traffic levels in the months ahead.”

The R.P.I.’s current situation index, which measures trends in four industry categories, including same-store sales, traffic, labor and capital expenditures, stood at 98.6 in January — down 1.1% from its December level. Thirty-nine per cent of restaurant operators reported a same-store sales gain between January 2010 and January 2011, down from 48% of operators who reported higher same-store sales in December. In comparison, 44% of operators reported a same-store sales decline in January, up from 35% of operators who reported lower sales in December.

The R.P.I.’s expectations index, which measures operators’ six-month outlook, stood at 101.8 in January — down 0.5% from December’s 45-month high of 102.4. Restaurant operators remain optimistic sales levels will improve in the months ahead. Forty-seven per cent expect to have higher sales in six months, compared with the same period in the previous year, down from 55% who reported similarly last month. In comparison, 14% 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 8% who reported similarly last month.