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# 2006 NRAA: Tweaks and Updates

As promised, I made some changes to Net Runs Above Average, with some help from Richard Wade and David Gassko. First off, let's start by recapping what it is that Net Runs Above Average does, for two reasons. The first being to introduce those who may not be familiar with it to the statistic itself. Secondly, it has been around for almost a year and I still haven't written a single definitive article for it.

Net Runs Above Average is meant to show offensive and defensive above average run value in one tidy number for easy analysis and inspection. It has rate forms and a cumulative form, and it is also easily adjusted to whatever you want to see with some simple arithmetic. Originally -- well, not originally, but within the last inception of it -- the components were primarily Equivalent Average and Rate2, both Baseball Prospectus statistics. EqA was an easy choice, as it is one of my favorite offensive statistics, and updated daily. Fans of Base Runs might take issue with that, and I am open to suggestion as always -- I try not to get too attached to any of this in the event that something better comes along.

Which brings me to my next point: we have converted over from Rate to Zone Rating, at the behest of more people than I expected. I had been thinking about converting over to something else, but I was not sure what exactly. After writing a the team previews this offseason, and seeing how many inconsistencies there were in Rate in comparison to other fielding systems out there, I decided that I might as well go through with it. Zone Rating seems to make the most sense, as it correlates well with some of the more accurate fielding systems out there, and is also available just as easily as Rate.

As far as the actual inputs into NRAA go, there is some explaining that needs to be done I think. To calculate the offensive runs above average, we use this formula (EQRAA is equivalent runs above average):

EQRAA = (5*OUTS*EqA^2.5)-(5*OUTS*.26^2.5)

The .260 might need some explanation. That is league average EqA for all batters. To calculate the defensive runs above average, we now tweak Zone Rating into something that Richard called "ZRate" (still listed as ZR in the table for space purposes). This is figured by calculating the chances and plays above average for a fielder at a given position, and then using Chris Dial's run value for an out for each position times the plays above average to figure the ZRate, or runs above average for that player. This number is then plugged into the spreadsheet where Rate used to be, and NRAA is born once all the fields are filled out properly.

Formerly, NRAA was normally in a per 100 game form, due to the use of Rate, which is also in that form. I was convinced about 15 minutes before writing this that converting to 150 games would make more sense, as it is a full season. I've switched over for now, and we'll see how it goes from there. Besides that, the only changes were in fitting the spreadsheet over to ZR rather than Rate.

The other version of NRAA that you will actually see more often is positionally adjusted Net Runs Above Average. NRAA is the raw form; pNRAA is the more accurate reflection of above average run value, due to its additional adjustments. The only real changes to pNRAA come in the equation for EQRAA:

pEQRAA =(5*OUTS*EqA^2.5)-(5*OUTS*pEqA^2.5)

pEqA replaces the .260, and that figure is whatever the average EqA for the position we are calculating is. This is possible now that BP has started to list the EqA's by position in their statistics reports, which makes my job a great deal simpler.

I have included a table in this article that lists the top 40 centerfielders according to pNRAA as of May 15. I am also including a spreadsheet attachment with the same numbers for sorting purposes, so people can play with it if they want to, as well as the ZRate sheet for center. I figure that this will be one of the easiest ways to get feedback and criticisms out there, since they most likely do a better job explaining the process than I can.