Duckworth–Lewis method

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In the sport of cricket, the Duckworth–Lewis method (D/L method) is a mathematical formulation designed to calculate the target score for the team batting second in a limited overs match interrupted by weather or other circumstances. It is generally accepted to be the most accurate method of setting a target score. The D/L method was devised by two English statisticians, Frank Duckworth and Tony Lewis.[1]

The basic principle is that each team in a limited-overs match has two resources available with which to score runs: wickets remaining, and overs to play. Where overs are lost, setting an adjusted target is not as simple as to reduce the batting team's run target proportionally, because a team batting second with ten wickets in hand and 25 overs to play can be expected to play more aggressively than one with ten wickets and a full 50 overs, and can consequently achieve a higher run rate. The Duckworth–Lewis method is an attempt to set a statistically fair target for the second team's innings, based on the score achieved by the first team, taking their wickets lost and overs played into account.

Examples

Stoppage in first innings

In the 4th IndiaEngland ODI in the 2008 series, the first innings was interrupted by rain on two occasions, resulting in the match being reduced to 22 overs a side. India (batting first) made 166/4. England's target was therefore set by the D/L method at 198 from 22 overs.

During the fifth ODI between India and South Africa in January 2011, rain halted play twice during the first innings. The match was reduced to 46 overs and South Africa scored 250/9. The D/L method was applied which adjusted the target to 268. As the number of overs was reduced in between South Africa's innings, this method takes into account what South Africa would have scored before the first interruption.

Both examples illustrate how the D/L method is applied. In the case of the first match, as England knew they had only 22 overs the expectation is that they will be able to score more runs from those overs than India had from their (interrupted) innings. England made 178/8 from 22 overs, and so the match was listed as "India won by 19 runs (D/L method)".[2]

Stoppage in second innings

A simple example of the D/L method being applied was the first One Day International (ODI) between India and Pakistan in their 2006 ODI series. India batted first, and were all out in the 49th over for 328. Pakistan, batting second, were 7 wickets down for 311 when bad light stopped play after the 47th over.

In this example, Pakistan's target, had the match continued, was 18 runs in as many balls, with three wickets in hand. Considering the overall scoring rate throughout the match, this is a target most teams would be favoured to achieve. And indeed, application of the D/L method resulted in a target score of 304 at the end of the 47th over, with the officially listed result as "Pakistan won by 7 runs (D/L Method)".

Examples in T20 matches

During the 2010 ICC World T20, the D/L method was used in the group stage match between Sri Lanka and Zimbabwe. Sri Lanka scored 173/7 in 20 overs batting first and Zimbabwe were 29/1 in 5 overs when rain interrupted play. Sri Lanka won the match by 14 runs according to the D/L method.[3]

During the 2012/13 KFC Big Bash League, the D/L Method was used in the 2nd Semi Final played between the Melbourne Stars and the Perth Scorchers at the WACA Ground. Melbourne scored 183/2 from 18 overs after a rain delay. Following a further rain delay, Perth returned to the field to face 13 overs, with a reduced target of 139. Perth won the game by 8 wickets following a boundary off the final delivery (A free hit).

The essence of the D/L method is 'resources'. Each team is taken to have two 'resources' to use to make as many runs as possible: the number of overs they have to receive; and the number of wickets they have in hand. At any point in any innings, a team's ability to score more runs depends on the combination of these two resources. Looking at historical scores, there is a very close correspondence between the availability of these resources and a team's final score, a correspondence which D/L exploits.[4]

Using a published table which gives the percentage of these combined resources remaining for any number of overs (or, more accurately, balls) left and wickets lost, the target score can be adjusted up or down to reflect the loss of resources to one or both teams when a match is shortened one or more times. This percentage is then used to calculate a target (sometimes called a 'par score') that is usually a fractional number of runs. If the second team passes the target, then the second team is taken to have won the match; if the match ends when the second team has exactly met (but not passed) the target (rounded down to the next integer) then the match is taken to be a tie.

An example of such a tie was found in the one day international between England and India on 11 September 2011. This match was frequently interrupted by rain in the final overs, and a ball-by-ball calculation of the Duckworth-Lewis 'par' score played a key role in the tactical decisions made during those overs. At one point, India were ahead according to this calculation, during one rain delay (and would have won if play was unable to be resumed). At a second rain interval, England, who had scored some quick runs (precisely because they were aware of the need to get ahead in terms of D/L) would correspondingly have won if play hadn't resumed. Play was finally called off with just 7 balls of the match remaining and England's score equal to the Duckworth-Lewis 'par' score, therefore resulting in a tied match.

This example does show how crucial (and difficult) the decisions of the umpires can be, in terms of assessing at exactly what point the rain is heavy enough to justify ceasing play. If they had done so one ball earlier, England would have been ahead on D/L, and so would have won the match (equally, if play had stopped one ball later, without England scoring off that ball, India would have won the match – indicating how finely-tuned D/L calculations can be in such situations).

Application

The Duckworth–Lewis method is fairly simple to apply, but requires a published reference table and some simple mathematical calculations. As with most non-trivial statistical derivations, the D/L method can produce results that are somewhat counter intuitive, and the announcement of the derived target score can provoke a good deal of second-guessing and discussion amongst the crowd at the cricket ground. This can also be seen as one of the method's successes, adding interest to a "slow" rain-affected day of play.

For 50-over matches, each team must face at least 20 overs before D/L can decide the game, and for Twenty20 games, each side must face at least five overs before D/L can decide the game.

History and creation

The D/L method was devised by two British statisticians, Frank Duckworth and Tony Lewis. It was first used in international cricket in the second game of the 1996—97 Zimbabwe versus England One Day International series, which Zimbabwe won by seven runs,[5] and was formally adopted by the International Cricket Council in 1999 as the standard method of calculating target scores in rain shortened one-day matches.

Various different methods had been used previously, including the average run-rate method, countbacks to the score that the first team had achieved at the same point in their innings, and targets derived by totalling the best scoring overs in the first innings.....

All these methods have flaws that are easily exploitable; for example, run-rate ratios take no account of how many wickets the team batting second have lost, but simply reflect how quickly they were scoring when the match was interrupted, so if a team felt a rain stoppage was likely they could attempt to force the scoring rate without regard for the corresponding highly likely loss of wickets, skewing the comparison with the first team.

An example of this is the third final in the 1988/89 Benson and Hedges World Series Cup between Australia and the West Indies. Rain stopped play for one hour and 25 minutes with the West Indies needing 180 off 31.2 overs chasing Australia's 4/226 off 38 overs, and the revised target left the West Indies needing 61 off the 11.2 overs that remained; the West Indies won the match (and the competition) with 4.4 overs remaining after Desmond Haynes hit Steve Waugh for six. Australian fans loudly booed this unsatisfactory conclusion, and criticism from the media led to the average run rate method being replaced by the most productive overs method for setting revised targets in interrupted matches.[6] In this match, the D/L method would have increased the target to 232 to take into account a two-hour rain delay during Australia's innings, and then revised the target to 139 after the second interruption.

The most notorious example of this is the "best-scoring overs" method used in the 1992 Cricket World Cup. In the semi-final between England and South Africa, rain stopped play for 12 minutes with South Africa needing 22 runs from 13 balls chasing England's 6/252 off 45 overs, and the revised target left South Africa needing 21 runs from one ball, which was a reduction of only one run compared to a reduction of two overs, and a ridiculous target given that the maximum score from one ball is generally six runs.[7] The D/L method avoids this flaw: in this match, the revised D/L target would have been four runs to tie or five to win from the final ball.[8]

The published table that underpins the D/L method is regularly updated, using source data from more recent matches. From the 1999 Cricket World Cup match in Bristol between India and Kenya, Tony Lewis noticed that there was an inherent weakness in the formula used at the time that would give a noticeable advantage to the side chasing a total in excess of 350. A correction was very soon built into the formula and the software to correct this, by including a 'match' factor. However, this minor correction was not fully adopted by users until the 2004 update. Updating the source data in its own right would reflect the overall trend that one-day matches were achieving significantly higher scores than in previous decades, affecting the historical relationship between resources and runs.

At the same time as this update, the D/L method was also split into a Professional Edition and a Standard Edition.[9] The main difference is that while the Standard Edition preserves the use of a single table and simple calculation – suitable for use in any one-day cricket match at any level – the Professional Edition uses substantially more sophisticated statistical modelling, and requires the use of a computer. The Professional Edition has been in use in all international one-day cricket matches since early 2004.

In June 2009, it was reported that the D/L method would be reviewed for the Twenty20 format after its appropriateness was questioned in the quickest version of the game. Lewis was quoted admitting that "Certainly, people have suggested that we need to look very carefully and see whether in fact the numbers in our formula are totally appropriate for the Twenty20 game."[10]

Criticism

The D/L method has been criticised on the grounds that wickets are a much more heavily weighted resource than overs, leading to the suggestion that if teams are chasing big targets, and there is the prospect of rain, a winning strategy could be to not lose wickets and score at what would seem to be a "losing" rate (e.g. if the asking rate was 6.1, it could be enough to score at 4.75 an over for the first 20–25 overs).[11]

Another criticism is that the D/L method does not account for changes in proportion of the innings for which field restrictions are in place compared to a completed match.[12]

More common informal criticism from cricket fans and journalists of the D/L method is that it is overly complex and can be misunderstood.[13][14] For example, in a one-day match against England on 20 March 2009, the West Indies coach (John Dyson) called his players in for bad light, believing that his team would win by one run under the D/L method, but not realising that the loss of a wicket with the last ball had altered the Duckworth-Lewis score. In fact Javagal Srinath, the match referee, confirmed that the West Indies were two runs short of their target, giving the victory to England.

Cultural influence

"The Duckworth Lewis Method" is the name of a band formed by Neil Hannon of The Divine Comedy and Thomas Walsh of Pugwash, which recorded a self-titled concept album of cricket songs.[15][16]

References

1. ^ "A Decade of Duckworth-Lewis". BBC. 1 January 2007. Retrieved 2009-03-21.
2. ^ Scorecard for the rain-affected 4th ODI between India and England on 23 November 2008, from Cricinfo.
3. ^ http://www.cricinfo.com/world-twenty20-2010/engine/current/match/412686.html
4. ^ Data Analysis Australia's detailed mathematical analysis of the Duckworth-Lewis Method daa.com.au.
5. ^ Scorecard of the 2nd ODI between England and Zimbabwe, 1 January 1997, from Cricinfo.
6. ^ http://www.espncricinfo.com/wisdenalmanack/content/story/150754.html
7. ^
8. ^ "Stump the Bearded Wonder", Bill Frindall explains how D/L would apply to 1992 WC semi-final
9. ^
10. ^ Duckworth-Lewis to review their formula for T20 matches
11. ^ Bhogle, Srinivas, The Duckworth/Lewis Factor, Rediff.com.
12. ^ Booth, Shane, quoted in For a Fair Formula, The Hindu.
13. ^ Varma, Amit, Simple and subjective? Or complex and objective?, ESPNcricinfo
14. ^ Charlie Brooker, AV campaigners have created a stupidity whirlpool that engulfs any loose molecules of logic, dismisses the claim of the simplicity by citing the method's formula, The Guardian, 25 April 2011. Retrieved 2011-04-28
15. ^ BBC news interview with The Duckworth Lewis Method
16. ^ Interview with band