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May 21, 2012

Baseball Prospectus News

Improving the Odds

by Colin Wyers

We’ve identified incorrect data in the playoff odds report. The purpose of this post is to explain what happened, announce that it’s fixed, and offer some technical notes on the process so readers can reassure themselves that it’s now working as it should.

At the heart of the playoff odds report is a database table that contains the current-season MLB schedule. We use that table for many other products as well. After the season started, we moved many of the other products to a different schedule table that went back through the entire Retrosheet era and also included additional data. The playoff odds were a straggler, because we were working on a project that would allow us to run them for previous seasons. (The adjusted standings have been similarly modified, and soon we’ll have adjusted standings available back through 1974 on the site.) So the changes were made to the new playoff odds codebase, and the old codebase was left running on the old schedule table. Unfortunately, at some point during the season, the old schedule table stopped updating properly.

We’ve subsequently cut the old schedule table entirely, and now all products are running off the new schedule table. The new playoff odds codebase is in place as well, but due to the amount of data that needs to be processed, it will be some time before we’re ready to offer historic playoff odds for past seasons. We have rerun the playoff odds for the entire season, so the one- and seven- day deltas are working off corrected data and thus reflect changes in a team’s odds over time, not the correction of mistakes. Playoff odds for past dates this season can be viewed by clicking on the Hit List and navigating to past editions.

Running the odds over the whole season allows us to verify that the error has been corrected as well. Looking at the predicted win percentage in the report, and compared to the average rest-of-season win percentage from the simulation, we see an average difference of .003. So the simulation is correctly incorporating the inputs, within a certain tolerance. (Because it’s a Monte Carlo simulation, we would expect to see some variation from predicted record due to randomness.)

The next question one might have is how we determine the expected rest-of-season win percentage. There are three inputs:

  • A team’s third-order win percentage to date,
  • Its projected rest-of-season win percentage in the depth charts, and
  • Its strength of schedule (in other words, its opponents’ expected win percentages).

The weighting used varies based upon how many games have been played. Given a team’s third-order and depth chart (DC) winning percentages, we figure its expected win percentage like so:

We do this for every team. To figure the odds of the home team winning each game, we add in home field advantage and use the odds ratio, like so:

That gives us the home team’s winning percentage for that game. We take the average of this for the listed expected win percentage, so the number you see on the site will not exactly reflect the EXP_WPCT produced by the formula above.

Then, for each iteration of the sim, we produce a random number between zero and one; if the number is below or equal to the winning percentage, the home team wins. Otherwise, it loses. Then we rank the final standings from each simulated season and figure out how many times a team won either its division or one of the two wild cards.

We’ve written several new queries to monitor this report and will be checking on it daily to ensure that problems do not recur.

Colin Wyers is an author of Baseball Prospectus. 
Click here to see Colin's other articles. You can contact Colin by clicking here

Related Content:  Playoff Odds

10 comments have been left for this article. (Click to hide comments)

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"We have rerun the playoff odds for the entire season, so the one- and seven- day deltas are working off corrected data and thus reflect changes in a team’s odds over time, not the correction of mistakes."

Are you sure this is correct? I just clicked over to the odds that say they were updated this morning at 7am. And despite the fact that the Red Sox were the only team in the Al East to win yesterday their, 1 day delta is listed as minus 3.5%. That seems....unlikely.

May 21, 2012 08:27 AM
rating: 0
BP staff member Colin Wyers
BP staff

There's more inputs going into the change between days than just the game results - the third-order wins are updated, the DCs are updated periodically, the weighting between the two are updated, and there's some random variation involved as well. I'll take a little time to dig into the simulation inputs and get you a more detailed response in a little while.

May 21, 2012 10:46 AM

I've been wondering about this too. The 1 day deltas have occasionally been the opposite of what you'd expect for some time. I chalked this up to, yeah, maybe the team won, but maybe it was a close win that didn't do them any favors. Plus the problems that they've acknowledged having.

But with the problems solved, and Boston's chances almost having to go up after yesterday, I'd love to hear about this.

May 21, 2012 09:16 AM
rating: 2

FYI, not sure if you're using the same report to populate the Daily Hit List but currently all teams are showing a 0% Playoff Odds.

May 21, 2012 10:30 AM
rating: 0
BP staff member Matt Kory
BP staff

Yeah, we're on that. Thanks.

May 21, 2012 10:53 AM

How accurate are the playoff odds? Do you re-run them for previous seasons and prove out that a team with a 75% chance of making the playoffs really makes the playoffs 75% of the time? Or do external factors, such as future injuries, trades, etc. change the real likelihood?

I ask this because I look at the Dodgers, who are now 74% to make the playoffs. In the past 7 days, they went 5-2 while the Giants went 4-3, as did the D-Backs. They put their MVP on the DL, and lost their starting 2b and 3b to the DL. And their odds went up 18.5%.

Curiouser and curiouser.

May 21, 2012 12:32 PM
rating: 1
BP staff member Colin Wyers
BP staff

We haven't done historic testing of the odds yet, because we don't have a significant body of them using the current methodology. As noted above, we've got code designed to run on past seasons now, but it's slow and it's going to take a while to run. Once we have that, I'll look at the accuracy and post an update.

May 22, 2012 17:40 PM

The playoff odds *have not changed in 3-4 days*. Review your product before declaring it fixed.

May 21, 2012 12:55 PM
rating: 2
BP staff member Colin Wyers
BP staff

Sorry, it looks like there was a problem that cropped up between me writing this at the end of last week and the article going up this morning - I didn't catch it because I was unavailable most of the weekend. It's been fixed now, and the hit list has been updated as well.

May 21, 2012 17:25 PM

So, am I reading this correctly? You estimate win probability for each game, and then do many sims of the remaining season. From the results of these many sims, you estimate the expected number of wins for each team, plus the probability of winning the division, winning the wild card, etc. In other words, given the current records and these estimated game win probabilities, you're not do a direct/exact calculation.

May 21, 2012 13:28 PM
rating: 0
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Inside The Park: Farew... (05/21)
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