Click Forensics, which provides tools to online advertisers and publishers so that they can reduce click fraud, has raised $6 million in its third round of funding. While click fraud rates have fallen off somewhat in recent years, it remains a significant problem, accounting for about 15.3 percent of overall clicks during the fourth quarter of 2009, according to Click Forensics’ data.

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The startup—which counts both ad networks like Adknowledge and advertisers like Progressive Insurance among its customers—measures the quality of all of a customer’s clicks so that they can determine which clicks they should charge or be charged for. Click Forensics says it will use the cash to build “out new audience verification products for the display advertising market.” Earlier this year, the company launched a beta of a product that lets display advertisers filter out “unwanted impressions.”