First, the good news: The number of click-throughs on mobile advertisements that were mistaken or fraudulent dropped slightly over the last year. Now the bad news: As much as 40% of clicks on mobile ads are so-called worthless clicks, offering no return on investment for the advertiser, according to a new study.

The study, which was released on Wednesday, was commissioned by Trademob, a Berlin-based mobile app marketing platform. The company analyzed six million mobile advertising clicks on 10 of the biggest mobile advertising networks. Conclusion: Advertisers are wasting a lot of money on mobile ads.

“There’s a problem when only 60 cents of every dollar are actually being put to work,” said Trademob founder and CEO Ravi Kamran.

The study was conducted in June and found that 22 percent of ad clicks are “misclicks,” or accidental clicks, and 18 percent were fraudulent. Those accidental and fraudulent clicks had a conversion rate of less than 0.1%.

There were some bright spots in the study for advertisers. A similar study conducted a year ago said 47% of clicks on mobile ads were worthless, so the percentage appears to have dropped. And the Trademob study showed a decline in the number of accidental clicks, from 25% to 22%.

“We are seeing fewer accidental clicks today due to increased device screen size and different user behavior,” Kamran said. “Still, more than one in five clicks is purely accidental, which can be prevented by proper ad placement and ad verification techniques.”

Harder to fight, however, is click fraud. Of the fraudulent clicks in the study, 56% involved sophisticated botnet and client-side fraud, typically using hijacked devices to create fake clicks that go unnoticed by the user. The other 44% were attributed to server-side fraud, in which publishers instructed servers to report data that masked millions of fake clicks as real ones.