3rd March 2015
One of the more common subjects for queries we receive at Sports-Reference is our SRS (Simple Rating System) figures. For some background, the first of our sites to add SRS was Pro-Football-Reference, when Doug Drinen added it to the site in 2006 and provided this excellent primer. The important thing to know is that SRS is a rating that takes into account average point differential and strength of schedule. For instance, the 2006-07 Spurs won games by an average of 8.43 points per game and played a schedule with opponents that were 0.08 points worse than average, giving them an SRS of 8.35. This means they were 8.35 points better than an average team. An average team would have an SRS of 0.0. The calculation can be complicated, but the premise is simple and it produces easily interpreted results.
However, there are some variations in the way we calculate SRS across our various sites. We'll break down these differences below.
Pro-Football-Reference.com SRS: PFR's SRS is unique in that a home-field advantage is included as a part of the calculation because of the short schedule compared to the other sports (we don't want a team to look relatively weak at the halfway point because they've only played 3 of their first 8 at home, for instance). This HFA fluctuates yearly based on game results, but it is generally somewhere between 2 and 3 points (2006 being an outlier, as you'll see). Below is a look at the HFA numbers we have used since 2001. If you'd like to calculate these HFAs yourself, just sum up every team's home point differential and then divide by the total number of games played across the league that season. This data can easily be found in the Play Index for each season:
- 2001: 2.0081
- 2002: 2.2461
- 2003: 3.5547
- 2004: 2.5078
- 2005: 3.6484
- 2006: 0.8477
- 2007: 2.8672
- 2008: 2.5586
- 2009: 2.2070
- 2010: 1.8945
- 2011: 3.2656
- 2012: 2.4336
- 2013: 3.1055
- 2014: 2.4883
College Football SRS: Our CFB SRS does not contain a home-field advantage element, but it does have some other quirks. Most importantly, we have capped the margin of victory considered for the formula. Due to the number of mismatches seen in college football, the maximum point differential a team can be credited with in a game is 24. We also credit all wins as a minimum of plus-7 margin of victory (so if you win by 1 point, it's treated the same as a 7-point win). The same logic is applied to losses, as well. One other wrinkle for CFB is that all non-major opponents are included as one team for the sake of the ratings.
College Basketball SRS: SRS for college hoops is straight forward (no HFA & no adjusted MOV), but one item to note is that games against non-major opponents are not counted in our calculations.
MLB, NBA & NHL: All of these SRS calculations are straight forward with no adjustments for HFA and no capping of MOV. It should be noted, however, that no special consideration is given for extra-innings, overtimes or shootouts, either.
We'll close with a quick rundown of the various merits and weaknesses of SRS, from Drinen's original 2006 post. These bullet points were created to describe the system used for NFL SRS, but many of the strengths and weaknesses can applied to the other sports, as well:
- The numbers it spits out are easy to interpret - if Team A's rating is 3 bigger than Team B's, this means that the system thinks Team A is 3 points better than Team B. With most ranking algorithms, the numbers that come out have no real meaning that can be translated into an English sentence. With this system, the units are easy to understand.
- It is a predictive system rather than a retrodictive system - this is a very important distinction. You can use these ratings to answer the question: which team is stronger? I.e. which team is more likely to win a game tomorrow? Or you can use them to answer the question: which of these teams accomplished more in the past? Some systems answer the first questions more accurately; they are called predictive systems. Others answer the latter question more accurately; they are called retrodictive systems. As it turns out, this is a pretty good predictive system. For the reasons described below, it is not a good retrodictive system.
- It weights all games equally - every football fan knows that the Colts' week 17 game against Arizona was a meaningless exhibition, but the algorithm gives it the same weight as all the rest of the games.
- It weights all points equally, and therefore ignores wins and losses - take a look at the Colts season. If you take away 10 points in week 3 and give them back 10 points in week 4, you've just changed their record, but you haven't changed their rating at all. If you take away 10 points in week 3 and give back 20 points in week 4, you have made their record worse but their rating better. Most football fans put a high premium on the few points that move you from a 3-point loss to a 3-point win and almost no weight on the many points that move you from a 20-point win to a 50-point win.
- It is easily impressed by blowout victories - this system thinks a 50-point win and a 10-point loss is preferable to two 14-point wins. Most fans would disagree with that assessment.
- It is slightly biased toward offensive-minded teams - because it considers point margins instead of point ratios, it treats a 50-30 win as more impressive than a 17-0 win. Again, this is an assessment that most fans would disagree with.
- This should go without saying, but - I'll say it anyway. The system does not take into account injuries, weather conditions, yardage gained, the importance of the game, whether it was a Monday Night game or not, whether the quarterback's grandmother was sick, or anything else besides points scored and points allowed.
Posted in Announcement, Baseball-Reference.com, Basketball-Reference.com, CBB at Sports Reference, CFB at Sports Reference, Data, FAQ, Features, Hockey-Reference.com, Pro-Football-Reference.com, SRS, Stat Questions, Statgeekery, Uncategorized | 2 Comments »
3rd March 2015
We have added a process to our database that correctly determines NHL Rookie status, for the purposes of using the Player Finder tools in the Play Index. For example, see the Player Game Finder, and Player Season Finder.
Now you can use those tools to do a search for true rookies, in addition to the variety of other data points you can search on.
Hockey-Reference continues to make improvements to these tools. We welcome your feedback.
Posted in Announcement, Hockey-Reference.com, Play Index | Comments Off on Rookie Designation Added
27th February 2015
Posted in Announcement, Baseball-Reference.com, Basketball-Reference.com, CBB at Sports Reference, CFB at Sports Reference, expire21d, Hockey-Reference.com, Olympics at S-R, Pro-Football-Reference.com, Statgeekery | Comments Off on Hiring: Analytics Consultant, Posted Feb 27, 2015
17th February 2015
With the retirement of Jason Giambi I've seen some talk from people that there are now no longer any MLB players older than them. Since I'm never one to pass up on the opportunity to make a relatively useless tool, I've added a quick Pro Football Reference page to find active NFL players as old or older than you are. Basically, we just need to hope that Adam Vinatieri keeps kicking.
Posted in Announcement, Pro-Football-Reference.com, Trivia | 2 Comments »
16th February 2015
We're happy to announce a new promotion that allows users to get a year-long Play Index subscription (or renewal) for only $15, which is less than half of the normal price ($36).
All you have to do is make a deposit of at least $15 at this DraftKings link. Please note that this promotion is only for first-time DraftKings users. If you're unfamiliar with DraftKings, it is a site which offers daily fantasy games for cash in all of the major North American sports leagues, plus some others. Once you make your deposit via that link, we will be notified and send you an email with a coupon code for the year-long Play Index subscription.
The Play Index is our baseball research tool, which allows subscribers to make custom searches through 100 years of box scores, splits, streaks and events. You can also make custom searches for any season stat line in MLB history. A full description of the Play Index and its tools can be found here. It is the most powerful baseball research tool available to the public.
If you're already a Play Index subscriber, you can still take advantage of this offer. Your Play Index subscription will simply be extended one full year from its current expiration date.
Posted in Announcement, Baseball-Reference.com, Play Index | Comments Off on Get Free Play Index Subscription with $15 Deposit on DraftKings
11th February 2015
On Thursday, February 12th around 9:30am we'll be switching servers for Baseball-Reference.com (moving from unitas to tris). Should be no downtime #dalemurphywilling.
Posted in Announcement, Baseball-Reference.com, expire2d | Comments Off on Baseball-Reference.com Server Move, 9:30am ET Thursday
11th February 2015
Just a note that we have added Box Plus/Minus (BPM) to our College Basketball site this week.
As outlined in its introduction to Basketball Reference, BPM is an advanced stat intended to measure a player's total contribution as reflected by advanced, context-dependent box-score metrics like USG% and AST%. It was developed for the NBA using regression techniques against a 14-year-long sample of historical Regularized Adjusted Plus-Minus (RAPM) data. BPM estimates the number of points contributed by a player greater or less than an average player, per 100 team possessions.
We're able to calculate BPM for seasons dating back to 2010-11 and it can be found initially on player pages in the 'Advanced' table, on our school season pages, also in the 'Advanced' table, and we've also added several advanced stats - including PER, Win Shares, and BPM - to the conference registers, along the right side of that page. (see below image). However, the best way to view BPM, as a sorted leaderboard, or according to any other criteria - is to use our Play Index search tools.
Again, our thanks to the creator of BPM, Daniel Myers, and to those whose work serves as a component. The methodology and logic of Box Plus/Minus (BPM) is discussed in our About section, and please note the section specifically for the NCAA.
(Note that the 'Advanced' tables on the player and schools pages have changed just a little, to accommodate the new stats. Individual ORtg and DRtg have been moved to the 'Per 100 Possessions' tables and Points Produced - the main component of ORtg - has been moved further to the left on the 'Advanced' table.)
* We have published BPM but not VORP for college basketball, unlike the NBA. Value over Replacement Player (VORP) owes its meaning and derivation to a market with salaried players and teams on an equal footing, and thus an easy-to-establish theoretical "replacement level", which doesn't exist or make sense for the NCAA.
Posted in Advanced Stats, Announcement, Basketball-Reference.com, CBB at Sports Reference | Comments Off on Adding Box Plus/Minus (BPM) to College Basketball
9th February 2015
Adam Wodon has joined Sports Reference today as a Managing Director for Hockey Reference. Adam will be working out of our Philadelphia office as our staff size has now risen to six. Adam brings a great deal of development experience and hockey knowledge to Sports Reference. Adam is the founder and managing editor of College Hockey News. Adam is an Isles fan through and through and also supports the Mets, Jets, and Nets making him the first diehard National League fan on staff. Adam is also on twitter at (@chn_AdamWodon).
Hans VanSlooten (@CantPitch) who had been working on the hockey site for the last 14 months will be taking over primary day-to-day development of Baseball-Reference.com.
Posted in Announcement, Baseball-Reference.com, Basketball-Reference.com, CBB at Sports Reference, CFB at Sports Reference, Expire30d, Hockey-Reference.com, Olympics at S-R, Pro-Football-Reference.com | 2 Comments »
9th February 2015
Playoff Box Plus/Minus (BPM)
We're now able to calculate Box Plus/Minus (BPM) and Value Over Replacement Player (VORP) for the playoffs, by extending the methodology used for the regular season, and calculating a team efficiency rating based on performance in the playoffs, adjusted for strength of playoff lineups (by using playoffs minute distributions and regular season BPM ratings). These ratings are interesting in themselves, and reflect some of the great playoffs runs - the 1996 Bulls' rating was +19.3, and the 2001 Lakers were +20.1.
The BPM figures themselves reflect the best individual playoff performances. LeBron's 2009 BPM of 18.2 is tops by a large margin, followed by Kareem's 1977 run and then the Jordan playoffs from 1989-1991.
(For a detailed explanation and further discussion see the "Playoff Box Plus/Minus and VORP" section of the write-up.)
Playoff VORP (and Pro-Rating the Regular Season)
As well as BPM, we can now calculate VORP for the playoff season. (If you need a refresher on VORP as a concept, please visit the original write-up when we introduced it to Basketball Reference). We calculate VORP for the playoffs similarly to the regular season - that is, [BPM - (-2.0)] * (% of minutes played) - and it should be interpreted identically. Again, that's as the number of points the player has produced over a replacement player, per 100 team possessions. The significance of VORP compared to BPM is that it's accumulative, rather than a rate stat, and thus accounts more literally for value provided, weighted as it is by minutes played, and expressed on a per-82 game basis*.
(* - Yes, this is new. We are now pro-rating VORP based on the number of games that a team has played during the regular season, and doing the same for the playoffs. One point of VORP is one point of full season team efficiency differential. This affects partial seasons, strike-shortened seasons, and the playoffs. This has significance for the playoffs, in particular - players on teams that go further or play in 7-game series will accumulate more because they have more opportunities.)
Some Minor Changes to the Equation
Please note that we've also updated the methodology, such that some players will have a slight variation in the BPM and associated stats. These were technical fixes to the regression methodology, which had the result of slightly changing the relative value of component stats. For example, blocks and shooting efficiency are two factors that are more highly favored by the revision and thus Anthony Davis saw the largest increase in BPM (+0.8), however most changes were of a much smaller magnitude. Please see the details of this revision in the write-up, under the "Updates" section.
Again, Basketball Reference extends its thanks to Daniel Myers, whose research the above represents. Also, please see the revised 'About BPM' write-up that details the methodology and is generally extremely useful for understanding the context of BPM and VORP, and the best ways to think about and use these stats.
Posted in Advanced Stats, Announcement, Basketball-Reference.com, CBB at Sports Reference, Playoffs | 2 Comments »
6th February 2015
We have recently added Cuban statistics and Arizona Fall League stats. Our other new addition is statistics for the Dominican, Venezuelan, Puerto Rican & Mexican Pacific Winter Leagues.
These leagues can all be accessed via our Minor Leagues section. On player's minors pages, the abbreviations are DOWL (Dominican), VEWL (Venezuelan), PRWL (Puerto Rican) & MXPW (Mexican Pacific). These leagues also feature batting leaderboards, so you can find outstanding performances like Kendrys Morales batting .404, and pitching leaderboards.
Posted in Announcement, Baseball-Reference.com, Data | Comments Off on Winter League Statistics Added