The Duckworth-Lewis Method
An introduction to the D/L (Duckworth/Lewis) method of resetting targets in interrupted one-day cricket matches by Frank Duckworth & Tony Lewis
The D/L method of resetting targets in rain-affected one-day cricket matches was trialled successfully during 1997 by the International Cricket Council(ICC), the ECB (England & Wales Cricket Board) and the Zimbabwe Cricket Union (ZCU). It has already been chosen for use in 1998 by the ECB, the ZCU and New Zealand.
The method is the invention of Frank Duckworth and Tony Lewis. Frank is a consultant statistician and editor of the Royal Statistical Society's monthly news magazine, RSS NEWS. Tony is a lecturer in mathematical subjects in the Faculty of Computer Studies and Mathematics at the University of the West of England, Bristol and chairman of the Western Branch of the Operational Research Society
Following the experience of the method's application in 1997 they have introduced a few modifications designed to make the method's use even simpler. This article provides a summary of the way the method works.
Contrary to the belief in some quarters, it does not require a degree in mathematics either to understand it or to use it! Although a purpose-written computer program is available to countries adopting the system to enable calculations to be performed speedily and accurately, this is not necessary. All calculations can easily be performed using nothing more than a single table of numbers and a pocket calculator. With a little practice there is no reason why anyone should not be able to calculate the revised targets more-or-less instantly. The authors firmly believe that the method is simple enough that it could be adopted for use at all levels of limited overs cricket
Basis of the method
The D/L method works using the notion that teams have two resources with which to make as many runs as they can - these are the number of overs they have still to receive and the number of wickets they have in hand. From any stage in their innings, their further run-scoring capability depends on both these two resources in combination. The single table gives the percentage of these combined resources that remain for any number of overs left and wickets lost. An extract of the over-by-over table is given in Table 1. (A ball-by-ball version of the table has also been produced to enable scorers to deal with instances when play is interrupted mid-over.)
When a match is shortened after it has begun, the resources of one or both teams are depleted and the two teams usually have different amounts of resource for their innings. In this case a revised target must be set. The D/L method does this in accordance with the relative run-scoring resources available to the two teams. If stoppages cause the team batting second (referred to here as Team 2) to have less resources available, as is more often than not the case, then their target will be revised downwards. If, on the other hand, as often happens when Team 1's innings has been interrupted, the stoppages result in Team 2 having more resources available, then their target is revised upwards to compensate for the extra resources they have at their disposal.
Table 1: Extract from the table of resource percentages remaining
Reading the table
The single table applies to all lengths of one-day matches from 60 overs-per-side downwards. [In 1997 there was a separate table for all lengths of matches from 60 to 10 overs per side.] Because 50 overs-per-side matches are by far the most common, the resources listed in the table are expressed as percentages of those available at the start of a 50 over innings. Thus when there are 50 overs still to be received and no wickets have been lost, the resource percentage available is 100%. 60 over innings start with a resource percentage of 107.1% compared to a 50 over innings and 40 over innings start with a resource percentage of 90.3% compared to a 50 over innings.
In order to determine the correct resource percentage the batting side has remaining at any stage of an innings, the number of overs left must be identified. This number of overs left, in conjunction with the number of wickets lost, is then used to read the resource percentage remaining from the table.
For example, suppose that after 20 out of 50 overs a team have lost 2 wickets. They have 30 overs left. From the table you will see that the resource percentage remaining is 68.2%.
Suppose now that there is an interruption in play and 10 overs are lost from the innings of the batting side. When play can resume there are only 20 overs left but there are still, of course, 2 wickets down, and the table now tells us that the resource percentage remaining is 54.0%. Thus the shortening of the innings has caused the team to lose a resource percentage of 68.2 - 54.0 = 14.2%.
Having started with a resource percentage of 100% and lost 14.2%, then if they complete their innings with no further loss of overs, they will have had a resource percentage available for their innings of 100 - 14.2 = 85.8%.
Applying the D/L method
The procedure for setting a revised target, which is the same for any number of stoppages at any stage of the match, is as follows.
Example 1: Premature curtailment of Team 2's innings
Team 2 had less resource available than Team 1 so their target must be scaled down by the ratio of resources, 72.5/100
Note : The above result is quite fair as Team 2 were clearly in a strong position when play was stopped and would very likely have gone on to win the match if it hadn't rained. Most other methods of target revision in use would, unfairly, make Team 1 the winners. The average run rate method gives 201 to win, the Current ICC method gives 227 and the parabola method gives 226. [Setting the target by the method of Discounted Total Runs - the Australian rain-rule - requires knowledge of the runs made by Team 1 from their most productive overs but the target would almost certainly be no lower than that required under average run rate and would probably be much higher so that Team 2 would very probably lose by this method as well.]
Example 2: Interruption to Team 2's innings
In an ECB Axa Life (Sunday) League match Team 1 have scored 200 runs from their 40 available overs and Team 2 lose 5 wickets in scoring 140 runs in 30 overs. Play is then suspended and 5 overs are lost. What is Team 2's revised target?
Team 2 had less resource available than Team 1 so their target must be scaled down by the ratio of resources, 79.2/90.3
Example 3: Interruption to Team 1's innings
In an ODI, Team 1 have lost 2 wickets in scoring 100 runs in 25 overs from an expected 50 when extended rain leads to Team 1's innings being terminated and Team 2's innings is also restricted to 25 overs. What is the target score for Team 2?
Because of the different stages of the teams' innings that their 25 overs are lost, they represent different losses of resource. Team 1 have lost 2 wickets and had 25 overs left when the rain arrived and so from the table you will see that the premature termination of their innings has deprived them of the 61.8% resource percentage they had remaining. Having started with 100% they have used 100 - 61.8 = 38.2%; in other words they have had only 38.2% resources available for their innings.
Team 2 will also receive 25 overs. With 25 overs left and no wicket lost you will see from the table that the resource percentage which they have available (compared to a full 50 over innings) is 68.7%. Team 2 thus have 68.7 - 38.2 = 30.5% greater resource than had Team 1 and so they are set a target which is 30.5% of 225, or 68.63, more runs than Team 1 scored. [225 is the average in 50 overs for ODIs]
Team 2's revised target is therefore set at 168.63, or 169 to win in 25 overs, and the advantage to Team 2 from knowing in advance of the reduction in their overs is neutralised.
Note: Most of the other target resetting methods in use make no allowance for this interruption. They set the target of 101 to win simply because both teams are to receive the same number of overs. This is clearly an injustice to Team 1 who were pacing their innings to last 50 overs when it was curtailed, whereas Team 2 knew in advance of the reduction of their innings to 25 overs and have been handed an unfair advantage. D/L allows for this by setting Team 2 a higher target than the number of runs Team 1 actually scored, as described above.
Updated in 1999
Comments can be made to the authors via email.