Ivan Nova had a rough day yesterday, giving up four runs and failing to pitch four innings. He allowed four free passes (three walks and a hit batsman) while only striking out one. It was his worst game of the season it terms of strikeout-walk differential, but he hasn't been good in this category for the whole year. A good way to quantify this is kwERA (strikeout-walk ERA), which is a Tom Tango metric that is scaled to ERA and only uses strikeouts and walks as the inputs (walks include hit batsmen and exclude intentional passes). The formula scaled to the 2011 American League run environment is 4.87 - (12*(K-BB)). Nova's kwERA of 4.83 is fourth-worst in the league better than only Tyler Chatwood, Kyle Drabek, and Wade Davis. The chart below shows the in-game kwERA for each of Nova's starts, this time scaled to 100 as the league average (with each number away from 100 representing a 1% difference from the average).