Source: Federal Reserve Bank of Chicago  

Release: Chicago Fed National Activity Index  

Units:  Index, Not Seasonally Adjusted

Frequency:  Monthly

A zero value for the index indicates that the national economy is expanding at its historical trend rate of growth; negative values indicate below-average growth; and positive values indicate above-average growth.

For further information, please visit the Federal Reserve Bank of Chicago's web site:
http://www.chicagofed.org/webpages/research/data/cfnai/current_data.cfm