Percentage Point

Percentage point is the unit of the difference between two percentages. A rate moving from 2% to 3% has risen by one percentage point and by fifty percent. Both are true, they describe the same event, and the choice between them is rhetorical.

The gap widens as the base rate shrinks. A treatment lowering a risk from two in a thousand to one in a thousand gives a 50% relative reduction, an absolute reduction of 0.1 percentage points, and a number needed to treat of one thousand. Coverage reaches for the first figure, decisions need the third, and the relative figure cannot be converted into either without the base rate — an omission rarely accidental, since the large number does its work through the Anchoring Effect.

The reverse framing minimises: three percentage points off a figure already at 60% reads as noise, though it may be most of the movement available. Neither framing lies by itself; presenting one without the base rate removes the reader's ability to check, which is why giving all three is a Plain Language obligation rather than a nicety.

Related traps. "Support fell 10%" is ambiguous between points and proportion, which is why finance uses the basis point — one hundredth of a percentage point. Percentage changes are asymmetric: a 50% fall needs a 100% rise to recover, so averaging them across periods overstates performance where a geometric mean would not. An average of percentages over groups of unequal size is not the overall percentage.

Choosing whichever framing flatters a number under scrutiny is Goodhart's Law applied to presentation rather than to the measure, and it rests on the base-rate blindness that also makes a test look conclusive under Sensitivity and Specificity. Checking it is usually one line of Fermi Estimation.