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

The 17-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, published yesterday, was prompted by concerns that click-fraud estimates of between 14 percent and 35 percent are causing 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 $90m (about £47m) 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.

Fictitious clicks are detected as a result of counting web page reloads on an advertiser's site as multiple clicks on its Google ad. These reloads can occur for various reasons, such as when a user browses deeply into the advertiser's website and then hits the back button until reaching the original landing page, the report states. It can also happen if users hit the reload button on their browser while on the landing page, or if they open a browser window, thus reloading the landing page.

Fictitious clicks can also occur due to what the Google engineers call "conflation across advertisers and ad networks". This refers to the counting of one advertiser's traffic in another advertiser's report, even if they are on different ad networks.

Having detected these problems, Google now wants to work more closely with these auditing firms, to help them reach more accurate estimates, the report states. So far, Google's attempts to collaborate with them have generally been met with deaf ears.

"These two problems are serious, and have resulted in significant inflation of click-fraud estimates from each of the click fraud auditing firms we examined," the report reads.

The Google report is available online here.