Microsoft commissioned the well-known lab AV-Comparatives to re-evaluate a recent test taking prevalence of samples into account. Microsoft has plenty of info on the prevalence of malware since every patch Tuesday the Malicious Software Removal Tool both deletes prevalent malware and reports a raft of non-personal information back to Microsoft. The returned telemetry includes what (if any) threats were neutralized, but also tells them the Windows version, the version of any installed antivirus software, whether that software is up to date, and more.

So based on prevalence, the effectiveness of different Anti-virus software was ranked. The new report applies weighting based on Microsoft's reported prevalence.

Baidu took the biggest plunge, from second place to 22nd. Why? Because while it didn't miss many samples, the ones it did miss were extremely widespread.