A strict number, with its margin.

TrueTen turns one video of your swing into measurements of your body's positions, scores each one against tour and amateur ranges, and combines them into a rating out of 10. Every number comes with a ± so you can see how much to trust it.

What's measured

Apple's body tracking finds your joints in every frame of the video, on your iPhone. The app then finds the six moments of the swing from your hands (address, takeaway, top, transition, impact and finish) and measures your positions at each one. Distances are in percent of your leg length, so the numbers work at any height and distance from the camera.

The ball, the club face and the club-head path aren't measured: the club is a blur near impact. Grip, ball position and alignment aren't measured either.

How the margin of error is worked out

Each measurement's ± combines four sources:

• Tracking noise, measured in your own clip: how much the joints jitter while you stand still at address, and through the swing.
• Timing: how much the value would change if a moment of the swing (the top, impact) were off by a frame. A higher frame rate makes this smaller, which is why the app films at 120 frames a second when the camera allows it.
• Camera drift: any movement of the phone or of a planted foot between frames.
• The method itself: a known limit for measuring each position from a flat video.

The phone's margins were then calibrated on test swings where the true positions are known exactly (rendered figures, filmed from different distances, heights and angles): each margin was widened until it contained the true value 90% of the time. Those test swings are rendered, not real people, and real-world testing is ongoing, so the margins may change as it's done.

The margin decides what counts. A position is a fault only when it's outside the range by more than its margin. When it's outside but within the margin, it's too close to call and goes on a watch list instead of your plan. A change between two swings counts as real only when it's bigger than both margins combined, which is how comparisons and the plan's re-film checkpoints judge progress.

What the app leaves out, and why

A measurement whose margin is too wide to judge is shown greyed out, with the reason, and left out of the rating instead of guessed. Some are always left out on the phone:

• Shoulder and hip turn. From one flat video, rotation reads far too small (a 94° turn read as 55° in testing), so it's never judged.
• Lead-arm straightness at the top and hands at impact. The arm crosses the chest and the tracking misplaces it.
• Posture held through impact on the phone. The hip height at impact is too erratic; standing up still shows through the head rising.

A swing with fewer than five measurements that can be judged gets no rating at all. A video imported from your Photos is checked for framing, angle and frame rate, and its margins are widened when it's less precise than a recording made in the app.

Why 10 is reserved for tour-level swings

Every measurement has two ranges: a narrow tour band, where tour players sit, inside a wider acceptable range, where good amateurs sit. A measurement scores full marks only inside the tour band, loses a little between the tour band and the edge of the acceptable range, and loses much more beyond it. Each score is the expected score over the measurement's margin, so camera noise is neither forgiven nor punished.

Every shortfall takes away a share of the rating, in proportion to how far off it is and how much that position matters, so several faults cost more than one. Two signs of the same problem (head and hip sway, for example) count once in full and once at half. The rating's own ± comes from re-scoring with each measurement moved within its margin.

So a 6 isn't a bad mark: it's an honest one, and the app's job is to show you the one change worth the most points.

Coaching information, not medical advice. Warm up first and stop any drill that causes pain.