SDI is bunk, unfortunately. SDI adds the results from other defensive systems together -- heck just about every defensive system you can think of is taken and all added together to form SDI. Some systems count some things that others do not count, but there is considerable overlap. Any stat that has overlap (or is counted in multiple systems) will thus be counted multiple times by the SDI index. For example, with SDI, one error is suddenly counted several times inside of various algorithms that are trying to put forth a theory as to what an error is actually worth. So rather than SDI figuring out what an error is ACTUALLY worth (picking the best theory), SDI simply takes every definition of an error's worth and adds them all together. Suddenly an error's value is inflated into the stratosphere. So if a player happens to be outside the norm on some trait that is counted in multiple systems, SDI explodes that value out. But what if a great, important trait is only counted in one or two systems, and the player excels at it? It's buried in the mess. SDI might be the most comical baseball statistic ever and I'm a bit amazed that the SABR guys aren't embarrassed by its existence.