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How accurate is phone timing, really?

An honest answer, including where it is worse than a hand time and why that still beats a stopwatch.

Every coach asks this within about thirty seconds, and the honest answer has three parts: what the phone measures, what it cannot, and where the error actually comes from.

A stopwatch measures a human, not a race

A hand time contains the timer's reaction to the gun and their reaction to the torso crossing the line. Reaction to a starting pistol averages around 0.20 seconds, and the finish is guessed early because the timer sees the runner coming. That is why sanctioning bodies add 0.24 seconds to a hand time before comparing it to an electronic one: the correction exists because the error is real, consistent and in one direction.

It is not that hand timing is imprecise. It is that it is precisely wrong, by about a quarter of a second, in the athlete's favour.

What the phone actually does

StatLap starts the clock on the sound of the gun and stops it when a runner crosses a line you set on the screen. Neither of those is a person deciding when to press a button, so the reaction-time error is gone entirely. What replaces it is a different, smaller error: how often the camera looks.

• At 30 frames per second, the finish is known to within about 33 milliseconds.
• At 60, about 17.
• At 240 (slow motion on a modern iPhone), about 4.

So the worst case is roughly a thirtieth of a second, against a hand time's quarter. The gap is not close.

Where it is worse

Two places, and it would be dishonest not to name them.

The first is the gun. A phone microphone hears the bang and also hears a hurdle falling, a car door, and somebody shouting. StatLap learns your gun's sound before the race and rejects the rest, but a starting signal it has never heard in a venue it has never been in is the piece most worth testing before you rely on it.

The second is the finish line itself. A phone sees a plane, not a beam. Set the line badly (too high, at an angle, or with the camera off to one side) and it will trip on an arm rather than a torso. That is a setup problem rather than a timing one, but a setup problem still shows up as a wrong number.

What it is not

StatLap is app-timed, not FAT-certified. Times from it are not automatically valid for records that require certified fully-automatic timing, and the site says so on every page that shows one. Anybody who tells you a phone is a certified FAT system is selling something.

For a practice, a dual meet, a time trial or a season of tracking whether somebody is actually getting faster, the question is not whether it matches a $6,000 system. It is whether it beats a person with a stopwatch. It does, by about an order of magnitude.

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