The National Federation of Independent Business (NFIB) released its small-business optimism index for the U.S. on Tuesday. The index dropped to 92.6 in March from 92.9 in February. It was the lowest level since February 2014.

6 of 10 sub-indexes declined last month, while 4 sub-indexes rose.

"The small business sector, which historically produced half of our private GDP and served as the "R&D" sector of our economy, is underperforming, doing little more than operating in maintenance mode. Slow economic growth is now just a result of population growth, more haircuts, retail customers, health care patients, etc. But there is no exuberance, no optimism and not much hope, the numbers make it clear," NFIB Chief Economist Bill Dunkelberg said.