DRS Explained: How the Decision Review System Works in Cricket
The complete explanation of the Decision Review System (DRS) — what technologies it uses (ball-tracking, edge detection, hot spot), how reviews are initiated and adjudicated, the 'umpire's call' concept that protects borderline decisions, how many reviews each team has per innings, what happens when a review is used correctly vs incorrectly, and the ongoing controversy around whether DRS has made cricket fairer or added new complications.
What DRS Comprises
The Decision Review System uses three core technologies. Ball-tracking (hawk-eye or equivalent): a predictive model that reconstructs the ball's actual trajectory after pitching and projects where it would have gone had nothing intercepted it — used to assess LBW 'would have hit the stumps' and 'pitched in line' criteria. UltraEdge (snickometer): audio and visual waveform analysis that detects the sound spike associated with ball-on-bat contact — used to assess caught-behind and bat-pad catches. Hot Spot (when available): an infrared imaging system that detects the heat friction generated by ball-on-bat or ball-on-pad contact — used as visual corroboration for edge detection. Some venues and ICC events do not use all three technologies; the ICC minimum standard for full DRS requires all three.
How a Review Works
When an on-field decision is made by the umpire, the aggrieved party (batting team for a dismissal they dispute, fielding team for a not-out they believe should be out) can challenge the decision. The fielding captain or the batsman signals 'T' with their arms to indicate a review request. The umpire then refers the decision to the TV umpire, who uses the available technology to assess the specific grounds of the challenge. The TV umpire communicates a verdict: 'out', 'not out', or 'umpire's call' (where the margin is too small to overturn the original decision — the decision stands but the review is not used up). If the original decision is overturned, the on-field umpire reverses their call. Each team has a limited number of unsuccessful reviews per innings.
Umpire's Call
The most controversial element of DRS is 'umpire's call' — a zone around the stumps (for ball-tracking LBW decisions) where the ball's projected path only partially intersects the stumps. If the ball is hitting only the edge of the stumps (within a threshold ICC defines as approximately 50% of the ball's width overlapping the stumps), the decision stands as called by the on-field umpire regardless of what the technology shows. The rationale: ball-tracking is a prediction model, not a certainty — in the zone where the ball is marginally hitting the stumps, the tracking's margin of error is large enough that the umpire's original judgment deserves to stand. Critics argue umpire's call undermines DRS's purpose by maintaining umpire decisions the technology shows should be different; defenders argue it appropriately acknowledges ball-tracking's uncertainty.
Frequently asked questions
Has DRS eliminated umpiring errors?
DRS has significantly reduced the rate of incorrect decisions in international cricket but has not eliminated errors. Ball-tracking remains a predictive model with acknowledged margins of error (the umpire's call zone exists precisely because the technology is not perfectly accurate). UltraEdge can produce false positives (audio spikes from pad or bat-side deflections that are not ball-on-bat) and false negatives (very faint edges not registering). Hot Spot requires specific types of contact (friction-based heat) to show a mark — edges that produce minimal friction may not appear on hot spot even when a genuine edge occurred. The combination of technologies improves accuracy substantially over the unaided umpire, but the umpire's call zone and technological limitations mean it is not infallible.
Can fielders review a stumping?
Yes — stumpings are reviewable under DRS. The TV umpire checks whether the ball was caught cleanly by the wicket-keeper before the stumps were broken, and whether the batsman's foot (or body) was grounded behind the crease at any point before the stumps were broken. A batsman who lifts their back foot momentarily and is stumped before it returns to the ground is legitimately out; a batsman whose foot was grounded (touching the crease) at the moment of the stumping is not out. The technology used for stumping review is primarily slow-motion ultra-high-speed camera footage — ball-tracking is not required for stumpings since LBW criteria are not involved.
Why did some cricket boards resist adopting DRS?
The BCCI (Board of Control for Cricket in India) was historically the most resistant major board to DRS adoption, declining to use it in India home Tests for several years after its introduction. The stated reasons included concerns about ball-tracking accuracy (particularly hawk-eye's predictive model for balls that pitched outside the off-stump line) and hot spot's reliability. Critics suggested the resistance also reflected concern about the commercial value of umpiring controversy in Indian cricket broadcasting. India eventually adopted DRS for their home series from around 2016 onward, and the ICC made full DRS mandatory for all international matches using approved technology from 2018. No major series now plays without at least basic DRS.