Watts will not convert to volts on their own
Anyone who has looked up a conversion knows the moment. You have watts, you want volts, and the calculator asks for amps. The conversion is not difficult. It is impossible without a third number, because watts are volts multiplied by amps, and one equation will not settle two unknowns.
That habit of mind is worth carrying outside electrical work. Most numbers quoted at you are missing their third term, and almost nobody asks for it.
A percentage is a division that threw something away
A rate is one number divided by another. Report the rate and drop what you divided by, and you have handed someone watts while expecting volts back.
Professional tennis is a clean place to watch this, because the records are complete and nobody in them is selling anything.
Take 116,382 completed best-of-three singles matches, played between January 2023 and September 2026. In each one a player lost the opening set. Across the whole set, 16.52% of them won anyway. One figure, with 116,382 observations underneath it.
Break the same matches out by how that first set finished and the single figure comes apart.
- 7-6: 21.92%, over 14,265 matches
- 6-4: 19.41%, over 25,546 matches
- 7-5: 18.89%, over 10,252 matches
- 6-3: 17.39%, over 25,970 matches
- 6-0: 7.18%, over 5,833 matches
Two of those rows describe the same margin
6-4 and 7-5 are both a two game gap, and they do not give the same answer. 19.41% against 18.89%. The gap is small, and it runs the way few people would guess. The longer set, the one that reached 7-5, is the harder one to recover from.
So “lost the first set by two games” is not a complete description of anything. Reducing a set to its margin throws away which games were played. The data can still tell the two apart. The summary cannot.
The same figure, measured over different counts
Now split the same matches by tour.
- ATP: 18.21% over 10,945 matches, 95% interval plus or minus 0.72 points
- Challenger W: 17.25% over 4,731 matches, interval 1.08 points
- ITF M: 15.18% over 27,415 matches, interval 0.42 points
The interval is the third term. It says how far the figure would move if the same kind of matches were drawn again. ATP and Challenger W sit about a point apart, and their intervals overlap by enough that the ordering between them is not safe to quote. ITF M sits below both by more than any of the three intervals, and that one is a real difference.
Strip the intervals out and all three become numbers in a row. A row of numbers invites a ranking it cannot support.
Three questions that cost nothing
How many cases was it measured over. A rate over 4,731 matches is not the same kind of statement as a rate over 27,415. Both print to two decimal places anyway.
What was discarded to produce the rate. Every summary throws something out. The 6-4 and 7-5 rows exist only because somebody kept the set score instead of the margin.
How far would it move on another sample. If nobody can answer that, the figure is a direction. Use it as one.
None of this needs statistics. It needs the reflex a conversion table teaches in ten seconds. A number arrives without the quantity it depends on, so you ask for that quantity first.
The figures above come from the Live Tennis API, a JSON service for developers. It covers live scores, fixtures, players and match events, with point-by-point history behind them. The data is published openly under CC BY 4.0, free to reuse with attribution.
Method: completed best-of-three professional singles matches on ATP, WTA, Challenger and ITF, 1 January 2023 to 14 September 2026. Retirements and walkovers excluded. Counted from the point-by-point tape, with nothing estimated.