Time.com has an interesting Q&A with Bill James. Here’s the part I found most interesting:
You often show that conventional baseball statistics aren’t as important as they appear. In the book, you write “every year that passes, the ERA (Earned Run Average) becomes a little more irrelevant.” Why is that?
The reason the ERA is becoming a little more irrelevant every year is that pitchers don’t pitch whole innings anymore. Relief pitchers anyway. If you go back to 1915, 1920, really, all pitchers pitched full innings 99% of the time. And you could measure a pitcher’s effectiveness by how many runs he allowed in those whole innings. But modern pitchers, in particular modern relievers, pitch portions of an inning. And in a situation where each pitcher pitches a portion of an inning, who you charge the run to becomes critical. And the rule on whom we charge the run to is so careless and sloppy that it doesn’t work. It often leads to pitchers having ERAs that do not reflect how they really pitch, either because the reliever allowed a bunch of runs to score that were charged to somebody else, or because the starting pitcher who left guys on base got hurt by it.