When I hear about a new baseball metric, my ears perk up. I recently stumbled on an article introducing a new baseball metric, weighted isolated power or wISO. As a metric, ISO attempts to measure a batter’s power by taking a similar formula to SLG and removing single base hits as to leave only the “power hits.”
Many of us know ISO as 
The new formula uses linear weights based on a particular year’s wOBA value. For example, the 2014 version of wISO would look something like this:
According to the authors the use of linear weights, “will value extra base contributions to the degree that they actually lead to scoring runs.”
Run Scoring Correlation
Intrigued by the statement, I ran some numbers based on team wISO from 1901 to 2013 excluding the strike years 1981 and 1994. See my article on how various metrics stack up against runs. Note: I standardized the wOBA scale for simplicity.
Indeed, the authors of wISO are correct. The coefficient of correlation or 
of standard ISO to runs scored is 0.61 whereas the of wISO is 0.63. Not a big difference but moving in the right direction.
You can go to my Github page to find the code R code for the plots above.
Considerations on Usage
The first thing to occur to me (after looking at the math) was how would one apply wISO? I’ve never looked to ISO as a run correlation. If I’m looking for a good power number to compare to runs scored I would normally go for SLG, with an of 0.89. For me, ISO has always been an external stat to be used only when I want to look at a batter’s “isolated” power.
The authors of wISO did a fine job. The main problem isn’t with wISO but ISO itself. It doesn’t correlate to runs scored as well as other metrics simply because scoring runs isn’t just about power. Some of the best baseball players in history weren’t power hitters (Pete Rose and Ty Cobb spring to mind), so why would a run scoring metric be based on power alone?
That being said, wISO is a useful metric to isolate and attempt to measure a hitter’s power; something I’ll keep in my back pocket for future use.