Production declines led to another drop in the nation’s economic activity in September, the Federal Reserve Bank of Chicago reported Monday.

The Chicago Fed National Activity Index slipped to –0.58 last month, from –0.49 in August. While the sales, orders and inventories category made a positive contribution to the index, it was outweighed by declines in industrial production and the manufacturing of construction and business supplies.

The index weighs 86 indicators of national economic activity. A zero value for the index means the economy is expanding nationally at its historical trend rate of growth. A negative value indicates below-average growth.