The National Federation of Independent Business (NFIB) released its small-business optimism index for the U.S. on Tuesday. The index rose to 93.8 in May from 93.6 in April.

4 of 10 sub-indexes increased last month, 4 sub-indexes fell, while 2 were unchanged.

"The bottom line is that without an empowered small business sector, the economy will grow at a mediocre pace," NFIB Chief Economist Bill Dunkelberg said.

"Politicians in Washington credit any insignificant growth in the economy to their policies, but realistically, it's the increase in the population. At this point, we should expect the same slow growth for the rest of the year," he added.