The Chicago Fed National Activity Index fell to a minus 0.08 in July, down from 0.06 in June. The shift points to an economy expanding at a below-average pace.

The oft-called "Goldilocks" index tracks how far the economy strays from its historical trend line, centered around a benchmark of zero ("just right"). Scores above zero indicate above-average growth, while negative readings show the economy is growing below trend.

The number represents a weighted average of 85 economic indicators classified into four groups.

July's weakness was primarily driven by the personal consumption and housing group, which swung to a minus 0.09 contribution after adding 0.04 the previous month.

Thirty-seven indicators improved from June to July, and 48 indicators fell.