For today's sabermetric primer I decided to take a look at IsoP, or Isolated Power.
Isolated Power can trace its roots in one form or another back to Branch Rickey and Al Roth in the 1950’s. The metric measures a batters power based on how many, and what type of, extra-base hits a player has amassed. Whereas slugging percentage counts all hits including singles, IsoP deals with only doubles, triples and home runs.
The most common formula for IsoP is (SLG - AVG), which is the same as (2B + 3B*2 + HR*3) / AB. However, when Baseball Prospectus uses IsoP for their PECOTA projections they give equal weight to doubles and triples since "extending a double into a triple is generally an indicator of speed, rather than additional power."
I agree with Baseball Prospectus’ reasoning so on redsoxstats.com the final formula I use is: (2B + 3B + HR*3) / AB.
In 2009 the American League IsoP was .156 with the National League checking in at .143. Albert Pujols led the majors with his .329 mark while running away from Carlos Pena who finished second at .306. Luis Castillo checked in with the lowest IsoP of qualified hitters at .037.
When you think about power and the Red Sox one name comes to mind, David Ortiz. Since joining the Red Sox in 2003, Ortiz has a collective .287 IsoP. However, heading into his age 34 season, Ortiz’s IsoP is on a 3-year slide coming off of his magical 2006 season.
It should be noted that Ortiz did have an IsoP over .250 in each of the final four months of last season after not cracking .100 in April or May. Hopefully consistency returns in 2010 as the Red Sox will need his bat, but banking on an aging slugger in a downward spiral is risky.