WASHINGTON — The National Restaurant Association’s Restaurant Performance Index fell below 100 in May, indicating the overall outlook for the industry was down. Standing at 99.9 in May, it was the first time in six months the R.P.I. fell below 100.

“Like the economy as a whole, the restaurant industry’s recovery hit a speed bump in May with same-store sales and traffic levels softening from recent months,” said Hudson Riehle, senior vice-president of the research and knowledge group of the N.R.A. “However, the overall economic fundamentals of the restaurant industry remain positive, which will likely lead to stronger performances in the months ahead.”

Overall, restaurant operators had softer same-store sales in May with 39% of operators reporting a same-stores sales gain, down from 50% of operators who reported higher same-store sales in April. In addition, during the month 40% of operators saw same-store sales declines, up from 31% of operators who had lower sales in April.

There was also a net decline in customer traffic during the month with 33% of operators reporting an increase in traffic, down from 38% who reported higher traffic in April. At the same time 41% of operators reported traffic declines in May, up from 35% reporting declines in April.

Restaurant operators also are not as optimistic about the six-month outlook for the industry as they used to be, as 41% of operators expect to have higher sales in six months, down from 55% who said the same in January. Also, 20% of operators expect their sales volume during the next six months to be lower than it was during the same period of the previous year, and this figure was up from 8% who were expecting lower sales volume in January.

Finally, operators are not as positive about the overall direction of the economy in coming months. Twenty-four per cent of operators said they expect economic conditions to improve in six months, down from 46% in January and the lowest level in more than two years. Also, 21% of operators said they expect economic conditions to worsen in the next six months, up from 8% who said the same in January.