Better same-store sales compared helped boost the National Restaurant Association’s Restaurant Performance Index to a four-month high in October.

The monthly index, designed to measure the health of the U.S. restaurant industry, scored 100.9 in October, up 0.7% from September and the highest level since June, according to a release from the Washington, D.C.-based association.

The index was above 100 for the eighth straight month, according to the release. Values above 100 indicate a period of industry expansion, with an index scoring below 100 pointing to expectations of contraction, according to the release.

“The RPI’s October gain was driven by broad-based gains in the index components, most notably solid improvements in same-store sales and customer traffic,” Hudson Riehle, senior vice president of the Research and Knowledge Group for the association, said in the release.

“Looking forward, restaurant operators are relatively optimistic about sales growth in the months ahead, though their outlook for the overall economy remains mixed,” he said in the release.

A majority of restaurant operators reported higher same-store sales in October, according to the release, with 54% of operators signalling a same-store sales gain between October 2012 and October 2013. That is up from 41% who reported higher sales in September. Operators who said they had lower sales in October accounted for 30% of the total surveyed; that is down from 40% in September.

Looking ahead, 36% of restaurant operators surveyed in October expect higher sales in six months. That is an improvement from 34% with that expectation in September.

There are mixed views of the direction of the economy, according to the index results. Those who expect the economy to improve account for 27% of operators, while 26% expect the economy to decline.