How the DRS Works: Cricket's Decision Review System Explained
A complete guide to the Decision Review System — what technology it uses, how teams challenge on-field decisions, what 'umpire's call' means, the ball-tracking prediction zone, edge detection, when you lose a review, and the debates around the system's accuracy.
What Is the DRS?
The Decision Review System (DRS) is a technology-assisted review process in international cricket that allows teams to challenge on-field umpiring decisions. Introduced at international level in 2009 (first used in a Test between India and Sri Lanka), it was designed to correct clear errors in the two most commonly disputed decisions: LBW appeals and caught-behind (bat-pad) dismissals. The system uses a combination of ball-tracking technology (to predict whether the ball would have hit the stumps), edge-detection audio analysis (to detect if the ball touched the bat), and ball-spin tracking to reconstruct the delivery's path.
Each team starts with two reviews per innings in Test cricket (one in T20Is and ODIs). When a fielding captain or a dismissed batsman wants to challenge a decision, they make a T-shape with their hands — the international signal to the umpires that they are requesting a review. The third umpire (the off-field umpire watching the replays) then assesses all available technology and either upholds the original decision or overturns it. If the review finds the original decision was incorrect (the ball was hitting the stumps, or there was an edge), the team retains their review and the decision is overturned. If the on-field decision is upheld, the challenging team loses a review.
Ball-Tracking Technology
Ball-tracking — licensed as Hawk-Eye in most international matches — uses six or more cameras placed around the ground to capture the ball's trajectory at multiple points along its path. From this data, Hawk-Eye reconstructs the ball's path before it reached the batsman and projects where it would have continued after the point of impact (i.e., whether it was heading for the stumps). The technology can measure trajectory to within approximately 2.5 mm accuracy for the post-impact prediction — though the uncertainty increases the further the ball is predicted past the impact point, which is why umpire's call exists.
The ball-tracking visualization shown on broadcast — the red tube showing the ball's path and the yellow tubes showing where the stumps are — is the basis for the LBW decision component of a DRS review. The key test: was the ball pitching in line? Was the impact in line? Was the ball hitting the stumps? All three must be satisfied for an LBW to be given out on review.
What 'Umpire's Call' Means
Umpire's call is applied when the ball-tracking prediction shows the ball hitting the stumps but only marginally — in a zone where the technology's uncertainty makes it unclear whether the ball was definitely hitting or definitely missing. The specific threshold: if the centre of the projected ball path overlaps with the outer edge of the stumps (rather than being definitively within the stump zone), the original umpire's decision stands — whether it was out or not out.
The umpire's call rule is controversial because it produces asymmetric outcomes: if the umpire gave not out and the ball is in the marginal zone, the review fails and the batsman stays in (umpire's call not out). But if the umpire gave out and the ball is in the same marginal zone, the review also fails and the batsman is out (umpire's call out). Critics argue this means the umpire's original guess still determines marginal cases, defeating part of the purpose of the review. Defenders argue that the technology's uncertainty in that zone genuinely does not justify overturning an umpire's judgment.
Edge Detection: Snickometer and UltraEdge
To determine whether the ball hit the bat edge (relevant for caught-behind decisions and some LBW reviews where an inside edge may have preceded the pad impact), the DRS uses audio analysis technology. The original Snickometer compared the audio waveform from a stump microphone with the ball's trajectory video, looking for sound spikes that coincided with the ball passing the bat. UltraEdge (or Hot Spot in some competitions) is a more sensitive version that shows the audio waveform in real time alongside the ball's position.
Hot Spot is an infrared camera system that detects friction heat from bat-ball contact — when the ball edges the bat, the friction creates a heat signature visible on the infrared image. It is not used in all competitions because it requires expensive equipment and a temperature differential to work effectively. In cold conditions or on synthetic materials, the heat signature can be too faint to detect. The ICC now relies primarily on audio-based edge detection in competitions where Hot Spot is unavailable.
Retaining a Review: The Neutral Outcome
There is a third DRS outcome beyond 'review upheld' and 'review lost': the neutral call, which occurs when the third umpire determines that umpire's call applies and the original decision stands. In this case, the challenging team does NOT lose their review — they retain it for future use. This rule (introduced in 2016) was added because it was considered unfair to penalise a team for challenging what turned out to be a genuinely borderline call that the technology could not definitively resolve.
Strategic Use of Reviews
Elite captains treat DRS reviews as tactical resources rather than simple dispute mechanisms. A captain who wastes both reviews early in an innings leaves the team with no recourse for the rest of the innings — any later LBW or caught-behind that is wrongly given cannot be corrected. The general convention: never review unless you are at least 60–70% confident the decision was wrong. The worst DRS mistakes occur when players review out of frustration rather than genuine belief in their case.
Some teams have developed 'review cultures' — established team-wide protocols for who decides whether to review (usually a combination of the captain, the dismissed/appealing player, and a team-mate with a better viewing angle). India was notable for years for declining to adopt DRS in home Test series, citing concerns about the technology's accuracy — they eventually adopted it fully in 2016. Their hesitancy was partly attributed to Sachin Tendulkar's opposition to the system, though this was never officially confirmed.
Frequently asked questions
How many reviews does each team get?
In Test cricket: two reviews per innings per team. In T20Is and ODIs: one review per innings per team. Successful reviews are retained; unsuccessful ones are lost for the rest of that innings. Reviews reset at the start of each new innings.
Can a batsman review their own not-out LBW decision?
No — only dismissal decisions can be reviewed by the batting team. If an umpire gives not out to an LBW appeal, the fielding team can review it. If an umpire gives out, the dismissed batsman can review. The batting team cannot review a not-out decision unless the batsman was not given out.
Is DRS used in all international cricket?
DRS is mandatory for ICC events and most bilateral Test series. It is not mandatory in all ODI and T20I series — some lower-ranked nations or bilateral series between boards that have agreed to waive it may not use it. It is never used in domestic cricket except for some franchise leagues (e.g., IPL began limited DRS usage in 2018).