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SABR Defensive Index

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#1 Doomtints

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Posted 10 January 2017 - 11:13 AM

 

This is a misanalysis.

 

In particular, it misstates how a blend of numbers usually works. If a vintner blends several very different wines together, ones that have little in common except that each of them has an alcohol content of about 8-16%, the fact that they share this common feature does not mean that the resulting blend has its alcoholic content "inflated into the stratosphere".

 

 

 

This is not how SDI works. It does not blend numbers at all, there are no "ratios" as you suggest. SDI literally adds the numbers from other defensive systems together. Think of a stock index. The Dow Jones reflects the price of every stock in its index added together. Now what would happen to the index if it started adding in IBM five extra times?

 

What if someone decided to add together the Dow Jones and the S&P indexes to make a new index? Many companies would be counted twice, other companies would be counted once. This would be complete rubbish and it would be mocked.

 

What if you are an accountant and count dollars from one division and dollars from another division, without realizing that both divisions have counted some of the same things? You would either be fired or you would go to jail, depending on how the result was used.

 

This is SDI, which is why it has "index" in its name -- it adds together every common defensive system it can get its hand on. It does not massage the result in any way. This is what indexes do. I think you could call the SDI application of indexing, "Poop."

Edited by Doomtints, 10 January 2017 - 11:27 AM.


#2 Sconnie

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Posted 10 January 2017 - 06:24 PM

This is not how SDI works. It does not blend numbers at all, there are no "ratios" as you suggest. SDI literally adds the numbers from other defensive systems together. Think of a stock index. The Dow Jones reflects the price of every stock in its index added together. Now what would happen to the index if it started adding in IBM five extra times?

What if someone decided to add together the Dow Jones and the S&P indexes to make a new index? Many companies would be counted twice, other companies would be counted once. This would be complete rubbish and it would be mocked.

What if you are an accountant and count dollars from one division and dollars from another division, without realizing that both divisions have counted some of the same things? You would either be fired or you would go to jail, depending on how the result was used.

This is SDI, which is why it has "index" in its name -- it adds together every common defensive system it can get its hand on. It does not massage the result in any way. This is what indexes do. I think you could call the SDI application of indexing, "Poop."

The key word below is aggregator. There has been significant debate on this site regarding the validity of aggregators of various sorts.

One could also argue aggregating play by play metrics would by their very nature amplify extremes.

http://sabr.org/sdi

"The SABR Defensive Index draws on and aggregates two types of existing defensive metrics: those derived from batted ball location-based data and those collected from play-by-play accounts. The three metrics representing batted ball data include Defensive Runs Saved from Baseball Info Solutions, Ultimate Zone Rating developed by noted sabermetrician Mitchel Lichtman, and Runs Effectively Defended based on STATS Zone Rating and built by SABR Defensive Committee member Chris Dial. The two metrics included in the SDI originating from play-by-play data are Defensive Regression Analysis, created by committee member Michael Humphreys, and Total Zone Rating"

Edited by Sconnie, 10 January 2017 - 06:28 PM.


#3 ashburyjohn

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Posted 10 January 2017 - 09:32 PM

This is not how SDI works. It does not blend numbers at all, there are no "ratios" as you suggest. SDI literally adds the numbers from other defensive systems together.

I asked F.X. Flinn, one of the members of the group that created SDI, to comment on this discussion. Here are his remarks:

 

 

 

For position players:

 

15% TZ + 25% DRS + 25% RED + 20% UZR + 15% DRA

 

All expressed in terms of Runs saved.

 

Different for P's and C's since some measures do not apply to them so their indexes are adjusted up

 

Some players have measures that all agree and some differ wildly.

 

For example, if DRS loves and DRA hates a player, SDI may be lower than another player with a merely good DRS and a good DRA, despite the larger weighting of DRS. If you are inclined to think along with DRS, then that outcome would not ring true for you; it would appear that a player you loved was not as good as the other player.

 

This is a blend. A weighted average (the percentages adding to 100). It's not merely a sum of uncorrelated numbers, measured in differing scales.

 

While there exists no definitive measure of defense, this is an attempt by various knowledgeable practitioners to achieve a consensus-metric reflecting today's best understanding. It's better than just stumbling around in the dark.

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#4 Doomtints

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Posted 10 January 2017 - 10:51 PM

 

The key word below is aggregator. There has been significant debate on this site regarding the validity of aggregators of various sorts.

One could also argue aggregating play by play metrics would by their very nature amplify extremes.

http://sabr.org/sdi

"The SABR Defensive Index draws on and aggregates two types of existing defensive metrics: those derived from batted ball location-based data and those collected from play-by-play accounts. The three metrics representing batted ball data include Defensive Runs Saved from Baseball Info Solutions, Ultimate Zone Rating developed by noted sabermetrician Mitchel Lichtman, and Runs Effectively Defended based on STATS Zone Rating and built by SABR Defensive Committee member Chris Dial. The two metrics included in the SDI originating from play-by-play data are Defensive Regression Analysis, created by committee member Michael Humphreys, and Total Zone Rating"

 

I am 100% familiar with what SABR has to say about SDI, being a SABR member myself. If you dig around enough on those pages you will find the SDI formula. It is exactly as i described it.

 

"Aggregator" is a misleading term, but one of the meanings of "aggregator" is "Index." So they are not lying by using that term, but nevertheless the term misleads people. Look up what an Index is if I did not explain it adequately. 

Edited by Doomtints, 10 January 2017 - 10:52 PM.


#5 Doomtints

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Posted 10 January 2017 - 11:07 PM

 

I asked F.X. Flinn, one of the members of the group that created SDI, to comment on this discussion. Here are his remarks:

 

 

 

 

This is a blend. A weighted average (the percentages adding to 100). It's not merely a sum of uncorrelated numbers, measured in differing scales.

 

While there exists no definitive measure of defense, this is an attempt by various knowledgeable practitioners to achieve a consensus-metric reflecting today's best understanding. It's better than just stumbling around in the dark.

 

Um, no. They took 5 different systems and added them together. They simply gave slightly more weight to DRS and RED than the others. The formula is right there in front of you. If they added them all together without the weighting, it would look like this:

 

20% TZ + 20% DRS + 20% RED + 20% UZR + 20% DRA

 

I think the percentages might be throwing you off.The percentages do not indicate "blending" it's actually "weighting." A blend would not count the same events multiple times, the algorithm would account for that with a "blend."

And none of this changes my point. Most of these systems count the same events. By adding them together, whether you choose to emphasize a couple of the systems or not, the result can only be obtuse and in the case of SDI, grotesque. SDI creates the opposite of a bell curve for players that are good or bad at whichever traits are counted the most in the 5 systems.

 

As for stumbling around in the dark, everyone knows why SDI was created and it was not to create a new defensive algorithm to supplant the others. Considering "the dark" is five other systems that they borrow from, there's not much need to flip on the light. 

Edited by Doomtints, 10 January 2017 - 11:12 PM.


#6 Sconnie

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Posted 11 January 2017 - 07:24 PM

I am 100% familiar with what SABR has to say about SDI, being a SABR member myself. If you dig around enough on those pages you will find the SDI formula. It is exactly as i described it.

"Aggregator" is a misleading term, but one of the meanings of "aggregator" is "Index." So they are not lying by using that term, but nevertheless the term misleads people. Look up what an Index is if I did not explain it adequately.

i was agreeing with you, and yeah I know what an index is. Just trying to further the conversation with references and not name calling.
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