A new system to assess employee performance its reportedly behind a large number of recent terminations at Yahoo (YHOO).

Yahoo’s internal message boards are seeing numerous posts by high level executives irritated by the firings, which are attributed to the “Quarterly Performance Rewiews” introduced by Yahoo CEO Marissa Mayer. The new evaluation system has led to the dismissal of in excess of 600 employees so far, AllThingsD notes.

The new review system puts workers on a modified bell curve, with managers assigning them performance “misses” or “occasional misses” under terms dictated by the system. Employees who received lower rankings at least twice have been fired in recent weeks. Anonymous postings on Yahoo’s boards complain that the system forces managers to assign “misses” to employees even when they feel the assessment isn’t warranted.

Mayer has defended the QPR system, which was deployed more than a year ago, suggesting that it is not being properly implemented and that it doesn’t force managers to give employees lower rankings.

Earlier this year, Mayer raised eyebrows when she ended Yahoo’s long-running practice of allowing employees to work form home.

Shares of Yahoo climbed almost 2% in Tuesday morning trading.