FIP and the importance of defense

I've touched on this before, but fielding independent pitching (FIP) is a stat that approximates what a pitcher's ERA would be based on outcomes that do not involve fielders - strikeouts, walks, and homeruns. The stat was created on the premise that a pitcher has little to no control over balls in play that are not home runs, and the rate of hits allowed on balls in play is a function of luck and the defense behind the pitcher.

To this end, FIP (and other defense independent pitching statistics) is a better way to project future pitcher performance than ERA, which depends largely on the defense of a pitcher's team. To demonstrate the importance of the defense played behind a pitcher, I took a look at the top 5 and bottom 5 teams in baseball at defensive efficiency for 2008, according Baseball Prospectus. I then looked the difference between FIP and ERA for these teams. If ERA is greater than FIP, I would expect that the team's pitchers had poor defense played behind him. If ERA is less than FIP, I would expect that the pitchers had good defense played behind him. The numbers next to each team are the difference between ERA and FIP, and the team's overall ranking in this category. Remember, a negative number indicates a team's ERA outperformed its FIP, and vice versa:

Top 5 Defensive Teams
1. Tampa Bay Rays: -0.41 (2nd)
2. Chicago Cubs: -0.22 (6th)
3. Toronto Blue Jays: -0.32 (4th)
4. Oakland A's: -0.18 (8th)
5. Boston Red Sox: -0.08 (12th)

Bottom 5 Defensive Teams
26. Seattle Mariners: 0.13 (23rd)
27. Colorado Rockies: 0.47 (29th)
28. Pittsburgh Pirates: 0.26 (26th)
29. Cincinnati Reds: 0.01 (14th)
30. Texas Rangers: 0.53 (30th)

This is not an exact science - I'm not implying there is a direct correlation between team defensive efficiency and difference between ERA and FIP. However, it appears there is some kind of relationship there. I'd need to look at historical statistics to draw some kind of definitive conclusion. Hopefully this isn't too confusing.

In 2008, the Mets' team ERA was 4.07, and team FIP was 4.32, for a difference of -0.24 (rounded). They were 6th in baseball in defensive efficiency. Do not underestimate the importance of defense - this difference between ERA and FIP would equate to about 39 runs for the 2008 Mets.