Google has published a report concluding that click-fraud incidence is lower than some auditing firms have estimated.

The17-page document, prepared by Google engineers, is titled How Fictitious Clicks Occur in Third-Party Click Fraud Audit Reports and states that some methods commonly used to measure click fraud are flawed.

The study was prompted by concerns that click-fraud estimates of between 14 percent and 35 percent are happening among Google advertisers, some of which in turn are reducing their ad spending, the report states.

The most serious auditing problem detected by Google engineers is something they call "fictitious clicks," which involves supposed clicks on Google ads that never happened.

"As an example, a single AdWords click may appear as five events in some reports, leading to the identification of these events as 'click fraud,' and the reporting of five fraudulent clicks," the report reads.

Click fraud occurs when someone clicks on a pay-per-click ad with a malicious intent.

For example, a company official may click on competitors' ads to increase the rival's ad spending. Or a publisher may click on his website's ads to increase his commissions.

In either case, the advertiser is paying for a click that will not yield a legitimate business lead.

Click fraud is a big problem for the search advertising industry, and in particular for Google, whose revenue comes almost entirely from pay-per-click ads.

Google recently settled for $90 million a class-action lawsuit brought against it by advertisers claiming click-fraud damages. However, several hundred advertisers opted out of the settlement and retained their right to sue Google separately in the future.

Google singles out three auditing firms - ClickFacts, Click Forensics and MordComm's AdWatcher - in the report as using flawed methodologies and approaches to identifying click fraud on behalf of Google advertisers.

ClickFacts chief strategy officer Mikhail Ledvich said that Google correctly identified flaws in the report from his company included in the study.

That report, from February, was done while ClickFacts' system was still in beta, or test, mode, he said. The system has since been fixed its system to avoid counting "fictitious clicks," he said.

He declined to give an estimate of click fraud in the Google ad network, but said ClickFacts believes the incidence in general ranges between 10 percent and 30 percent.

Click Forensics and MordComm didn't immediately reply to requests seeking comment.