The Decision Review System lets players challenge an on-field umpire's decision using technology. It has been in international cricket since 2009 and has changed how the game is officiated, how batters play, and how much shouting happens on television.
How a review starts
Either side can ask. The fielding captain reviews a not out decision they think should have been out. The batter reviews an out decision they think should not have been.
The signal is a T made with both arms, and it has to be made within about fifteen seconds of the original decision. That limit exists to stop sides waiting for the crowd or the dressing room to tell them what to do, and it is enforced strictly.
Each team gets a set number of unsuccessful reviews per innings. Get it right and the review is retained; get it wrong and one is gone. Running out of reviews with an hour of the innings left is a real tactical cost, which is why captains guard them and why using one in frustration is treated as a mistake.
The technology
- Ball tracking. Multiple cameras follow the delivery and a model predicts where it would have continued after hitting the pad. This is what decides leg before wicket reviews.
- Edge detection. Stump microphones pick up sound as the ball passes the bat, displayed as a waveform, so a spike at the moment of passing indicates contact.
- Thermal imaging. A heat signature at the point of contact, used in some competitions as a second opinion on faint edges.
- Slow motion and ultra motion cameras. For catches, run outs, stumpings and whether a bowler overstepped.
The third umpire works through these in a fixed order, and the sequence matters. For a leg before review the question is asked in stages: was there an inside edge first, did the ball pitch in line, did it strike the pad in line, and would it have gone on to hit the stumps.
Umpires call
This is the part nobody agrees about.
Ball tracking is a prediction, not a measurement, and predictions carry uncertainty. So the system defines a margin. If the tracking shows the ball only marginally clipping the stumps, the technology is treated as not confident enough to overturn a human being, and the original decision stands whichever way it went.
The consequence is that two identical deliveries can produce opposite outcomes depending on what the on-field umpire said first. Given out and marginally hitting: out. Given not out and marginally hitting: not out. Same ball, same tracking, different result.
Supporters of the rule point out that this is exactly what a confidence margin is for, and that the alternative is treating a model's output as fact to the millimetre. Critics point out that the rule makes the umpire's original opinion decisive in precisely the cases where technology was supposed to help. Both are right, which is why the argument has not ended.
What it changed
Fewer obviously wrong decisions survive, which was the point. Batters are given out leg before far more often than they once were, because ball tracking demonstrated that deliveries which were traditionally assumed to be passing over the stumps or down the leg side often were not.
It also changed behaviour at the crease. Playing forward with the pad as a first line of defence is far riskier than it used to be, and the reduced use of that technique is one of the clearest technical shifts in modern batting.
Results in our archive record the decision as it finally stood. If you are reading a Test scorecard from the last fifteen years, some of those dismissals were reviewed, and the scorecard will not tell you which. That is what the highlights are for.
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