As the federal government pumps billions of bonus dollars into private Medicare health plans to encourage better care, the quality rating system used to award the bonuses is coming under increasing fire. Critics, including the Government Accountability Office and the Medicare Payment Advisory Commission, question whether the $8 billion-plus program is mostly rewarding mediocre patient quality. The latest attack came in a report last week from the American Action Forum, a conservative think tank, which argued that some standards are difficult to measure and that it's hard to score well when plans don't always know the criteria in advance.