Video review boundaries

Why did adding video review, which settles exactly what happened, make offside decisions feel more arbitrary than the linesman's guess ever did?

The complaint about linesmen was that they were guessing. So the guess was replaced with cameras, freeze frames and drawn lines — and the decisions became more controversial, not less. That is a strange result. If the old objection was inaccuracy, then accuracy should have dissolved it. Since it did not, the old objection was probably not what people thought it was. What was actually wrong with the linesman's call, and did the cameras fix that thing or a different one?

Begin with the quantity being decided. The offside law compares two positions at one instant, and the separation between them is continuous — any distance at all, including distances far smaller than anyone can see. So the law applies a binary threshold to a continuous variable, with nothing in between.

Now ask what happens near that threshold. The rule contains no tolerance. It does not say "clearly ahead"; it says ahead. So a margin of a metre and a margin of a centimetre are treated identically, and a procedure precise enough to resolve a centimetre will be obliged to.

Is a centimetre resolvable? Consider what the review actually measures. It has to fix the moment the ball is played, and it does that by choosing a video frame. At a fifty-frames-per-second camera — a common broadcast rate, from memory — consecutive frames are twenty milliseconds apart. A forward moving at eight metres per second travels sixteen centimetres in that gap. So the choice of frame alone carries an uncertainty far larger than the margins routinely being adjudicated. Higher-rate dedicated cameras and limb tracking shrink that figure, but they do not remove it: the ball leaves the boot over an interval, not at a point, and something must still decide when contact counted.

There is more than one such choice. Which body part counts — and which point on it. Where the plane through the defender sits when the camera is not level with it. How thick a drawn line is on the screen. Each is a decision made inside the procedure, and each carries its own error. Stack them and the total uncertainty is of the order of tens of centimetres.

So here is the first half of the answer: the measurement is not, in fact, sharper than the margins it is being asked to decide. It is sharper than the human eye, which is a different claim.

Now the second half, which is the more interesting one. The linesman was also wrong at those margins — almost certainly wronger. But his output was a flag, and everybody understood a flag as a judgement made in a second by a person running sideways. Error was priced in. The review's output is a picture with a line drawn on it, and a picture of a line does not look like a judgement. It looks like a measurement. So the same residual uncertainty that was previously absorbed as "the official's call" is now displayed as though it were a fact, and any remaining error reads as an injustice rather than as the cost of having officials at all.

Which raises the sharper question: was the arbitrariness ever increased, or only made visible?

The natural misreading here is that this is a story about vagueness in the law's language, of the kind that troubles words like "heap" or "reasonable". It is not. The offside rule is already a bright line: the concept is sharp and the boundary is exactly specified. The trouble arrives from the other direction — from a measuring apparatus whose own error is larger than the distances it is being asked to rule on, applied to a rule that was written when nobody could resolve those distances and so never needed to say what tolerance it would accept.

That distinction gives the account a test. If the diagnosis is right, then writing an explicit tolerance into the rule — deciding offside only when the margin clearly exceeds the measurement's own error, and otherwise letting the on-field decision stand — should reduce the sense of arbitrariness without changing what the law is for, since preventing goal-hanging never depended on centimetres. If a tolerance were introduced and the disputes continued unchanged, this explanation would be wrong, and the real cause would lie elsewhere — in the delay, perhaps, or in the loss of a single visible authority.

A natural comparison sits inside the sport already. Goal-line technology settles a threshold question of the same kind and has not generated the same resentment — because it needs almost none of these choices: one rigid object crossing a painted line, with no separate instant to sample and no debate about which part counts. It makes one judgement rather than four. That contrast is the strongest evidence for the account, and if goal-line calls were as contested as offside calls the account would be in trouble.

• What an explicit margin of error would look like written into a law, and who would then own the residual call.
• Why the same technology in cricket keeps an "umpire's call" band, and whether that band has been accepted or merely tolerated.

Every term the collection defines is gathered in the glossary.