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Just how worried are start-up investors?

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Venture capitalists are officially nervous.

How nervous?

A quarterly survey released on Thursday that measures sentiment among venture investors indicates that confidence fell by the most in five years during the latest three-month period.

Inflated private market valuations that don't reflect reality coupled with public market volatility pushed the Silicon Valley Venture Capitalist Confidence Index down to 3.39 from 3.73. The report, published by University of San Francisco entrepreneurship professor Mark Cannice, measures confidence on a five-point scale, with one as the lowest and five as the highest.

"The level of risk that late-stage private investors are underwriting is unprecedented and, arguably, unwarranted in many cases," said Karan Mehandru, a partner at Trinity Ventures, in the report.

Fundraising has been going gangbusters. According to the National Venture Capital Association, U.S. start-ups raised $47.2 billion in the first nine months of 2015, almost as much as all of last year. Numbers haven't been this big since the dot-com bust year of 2000.

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At the same time, there's much more chatter about the problem with negative gross margin businesses, high burn rates and troublesome financing terms.

It's the third straight quarterly dip in confidence and the steepest since the second quarter of 2010. The Index, which incorporates responses from 31 Bay Area venture capitalists, is also at its lowest since the end of 2011.