How DRS Works Guide

Cricket asks two people standing twenty metres away to judge, in real time and once only, whether a ball travelling at speed brushed the edge of a bat, or whether it would have hit the stumps had a pad not been in the way. Nobody gets all of those right. The Decision Review System exists to remove the clearly wrong ones, not to re-open every close one, and that distinction explains most of what people find strange about it.

Hold on to this: the on-field umpire still makes the decision, and the review is a check on it, not a replacement for it. A replay on the big screen changes nothing, and nor does the commentary box or the crowd. Only two things can change an on-field call: a review asked for by the players, or one of the third umpire's own checks. And the third umpire overturns only where the evidence is conclusive. Where it is thin, the original decision stands.

There are two separate routes to the third umpire, and they are easy to confuse. A player review is asked for by the batter who has just been given out, or by the fielding captain when an appeal has been turned down, and is signalled by making a T shape with both arms. It has to be asked for within fifteen seconds of the umpire's decision, before the players have moved on. That short window stops a captain from looking at the dressing room and reviewing on someone else's information.

An umpire review is the on-field umpires referring something they cannot judge themselves: run-outs, stumpings, whether a low catch carried, whether a fielder saved a boundary. These are not limited in number and do not come out of either side's allowance.

Player reviews are for dismissals only. You cannot review a wide, and you cannot review the umpire's decision not to call a no-ball. Front-foot no-balls are handled separately: where the technology is in place, the third umpire watches the bowler's landing foot on every delivery and tells the on-field umpire when it has gone over the line. That check belongs to the officials, not the players. A no-ball can still surface during a review, because before confirming any dismissal the third umpire checks the delivery was legal.

Ball-tracking. Several high-speed cameras follow the ball from the bowler's hand to the moment it strikes the pad. Up to that moment, the line you see is a record of where the ball really went. Past it, the line carrying on to the stumps is a projection: an estimate of where the ball would have gone had the pad not been there. Nobody filmed that part. It is worth restating, because the graphic looks identical either side of impact and viewers read the whole thing as footage.

Edge detection through sound. A directional microphone in the stumps is displayed as a sound trace lined up frame by frame with the pictures. A spike at the exact frame where bat and ball are level suggests contact; a spike a frame early or late is usually bat on pad, or the batter's foot landing. The timing is the whole of the evidence.

Edge detection through heat. An infrared camera picks up the warmth friction leaves behind, so a faint touch shows as a bright mark on the edge of the bat. It is strong evidence when it is there and weak evidence when it is not, because a fine edge does not always leave enough heat to see. Not every broadcast carries it, which is why DRS is not identical in every series.

Replays. For catches, run-outs and stumpings there is nothing to project. The question is what a single still frame shows: whether the fingers got under the ball, whether the bat was grounded at the instant the bails were dislodged. Slow motion and zoom, no estimates.

Almost every argument about DRS is an argument about lbw, and almost every argument about lbw is an argument about umpire's call. Ball-tracking answers three separate questions, in order, and a batter is only out if all three go against them.

• Pitching. Where did the ball land? A ball that pitched outside the line of leg stump cannot be out lbw, whatever happened afterwards.
• Impact. Where did it strike the batter, relative to the stumps? A batter struck outside the line of off stump while playing a genuine shot is not out.
• Wickets. Would the ball have gone on to hit the stumps?

Each of those answers carries a margin of uncertainty, because the part of the path that matters is an estimate rather than a recording. The system reports its answer with that margin included. When the projected path only clips the margin instead of clearing it, the result is umpire's call: the third umpire has not found conclusive evidence either way, so the decision the umpire made in real time is what stands.

Here is the consequence that upsets people. Exactly the same tracking picture can produce opposite results: a ball shown just clipping leg stump is out if the umpire raised his finger, and not out if he did not. That is not a flaw in the system, it is the entire design. The umpire decides, and the review only overrules when it is sure. The compensation is that an umpire's call outcome does not cost the reviewing side its review, because the honest answer was that it was too close to tell.

Each side gets a small fixed number of unsuccessful reviews per innings, with more allowed in the long form than in limited-overs cricket. The exact figure has moved more than once, including a temporary increase during the pandemic, when travel restrictions meant local umpires with less international experience were standing. Take the broadcast graphic as the authority for the match in front of you. The shape of the rule matters more than the figure, and the shape has been stable:

• The count is of failed reviews. A successful review is not deducted, so a side that keeps getting them right never runs out.
• An umpire's call outcome is not deducted either.
• Umpire reviews and the automatic no-ball checks never touch a side's allowance.
• The allowance is per innings and does not carry over, so reviews left unused when an innings ends are simply gone. In the longest form the allowance has at times also been topped up part of the way through a long innings.

On the field the change is real and mostly invisible. Umpires give marginal decisions knowing they may be examined, batters can no longer stand their ground hoping a thin edge goes unheard, and fielding sides cannot appeal at everything on the chance that one lands.

The louder claim is that DRS changed the lbw itself: that once ball-tracking showed how often a ball striking the front pad really would have hit, umpires became readier to give them. Our Test archive lets us look at the share of dismissals that were lbw, decade by decade.

DRS reached Test cricket in the late 2000s and spread unevenly through the 2010s, with some boards adopting it years after others, so the decades to compare are the 2000s and the 2010s. The lbw share went from 17.1% to 16.8%. It did not rise. It fell very slightly, by less than the gap between the 16.6% of the 1990s and the 17.1% that came before DRS existed, and the part-decade since sits at 16.2%, in the same narrow band. The share barely moved, and that is itself the finding. The interesting jump is a generation earlier: lbw went from 11.8% of dismissals in the 1970s to 15.5% in the 1980s, a bigger move than anything either side of DRS.

Be careful about what this can prove, which is very little. A share of dismissals is only a share: caught accounted for 61.6% of the 2010s, so lbw competes with everything else on the card. And nothing here separates DRS from everything else that changed over the same decades: pitches, the balance of pace and spin in attacks, the rise of spin in some eras, playing conditions. The honest reading is narrow. Whatever DRS did to individual decisions, and it certainly corrects some that used to stand, it did not visibly move how often Test batters end up lbw. The last row is a part-decade too: the archive's most recent Test is dated 2024-03-30.

A review is a resource with a hard limit and no carry-over, and you have seconds to spend it, so settle these before the match rather than during the appeal.

Ask who actually has evidence. The striker knows better than anybody whether bat touched ball. The non-striker has the best view of where the ball pitched. The keeper and the slips heard it. The bowler, running in and appealing on instinct, is the least reliable witness on the field and usually the loudest. Agree in advance whose word decides.

Geometry decides lbw reviews more often than conviction does. The further down the pitch the batter was struck, the longer the projected path has to run and the wider the margin of uncertainty, so the likelier the answer comes back as umpire's call. A batter beaten on the back foot, struck low and close to the stumps, is the review worth spending. One struck well forward and high on the pad, off a tall bowler getting bounce, usually is not.

Pitching outside leg is the one clean win. It is a plain not out however good the rest of the picture looks, so a batter who is certain the ball drifted down the leg side has a better case than a fielding side hopeful about height.

Weigh the match situation, not just the delivery. A review still in hand when the innings ends bought you nothing, so hoarding has a cost too. Late in an innings, with a set batter in and only the tail to come, protecting that batter is worth more than it was an hour earlier. And do not review the crowd: noise, a raised arm from square leg and a bowler's certainty are not evidence of anything.

Reviews are one of the handful of real decisions a captain owns, alongside batting first and declaring. Our guide to Test captaincy sets them side by side. Then take the armband and spend the reviews yourself in The Follow-On.

• MCC Laws of Cricket, Law 36: leg before wicket
• ICC playing conditions for international cricket: the Decision Review System
• CricArcade Test match archive
• Figures computed with scripts/build-guide-facts.ts over the archive's scorecards

• How to Read a Cricket Scorecard: A plain-English guide to the batting card, bowling figures, extras, fall of wickets and the result line, with what every abbreviation actually means.
• How Duckworth-Lewis-Stern Works: Why rain does not cost both sides the same, how overs and wickets in hand become a revised target, and what the method cannot know.
• Fielding Positions Explained: The map of a cricket field, from slip to deep backward square leg, and the modifier system that lets you decode a position you have never heard named.
• Economy, Strike Rate and Average: Reading Bowling Figures: The four numbers on a bowling line, the three rates built from them, and the scorecard notation that catches everyone out: 51.2 overs is not 51 and a fifth.
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