Nasdaq OMX Group Inc Chief Executive Robert Greifeld said the quality of companies trying to sell their shares to the public is improving as the long-shuttered IPO market returns to life.

IPO levels have not returned to the levels we saw in '06 and '07, and will not in the short term, but the rebirth starting in April with the IPO of OpenTable on NASDAQ appears to be accelerating, Greifeld said at the Boston College Chief Executives Club.

And critical investors, who were burned by last year's financial crisis, are helping ensure that only very strong companies list their stocks, he added.

We will see the quality of initial public offerings get higher, Greifeld said. You have a very cynical investor base out there, so you have to be good, he added.

Five companies are expected to go public this week alone, including online video game company Shanda Games and lithium-ion car battery maker A123 Systems Inc. They are part of a surge of applications that has come in the last two months, Greifeld said.

Companies backed by venture capitalists are especially positive for the economy Greifeld said, noting that many of these companies are truly innovating, changing lives, and generating jobs.

(Reporting by Svea Herbst-Bayliss, editing by Matthew Lewis)