GlossaryStatistics and interpretation
Percentage-point change
Percentage-point change subtracts one percentage from another. Learn why it differs from relative percentage change, record counts, and rate units.
Percentage-point change is the arithmetic difference between two percentages. Subtract the earlier percentage from the later percentage and label the result in percentage points. It describes how far the percentages moved on the percentage scale, rather than the relative percentage increase or decrease in the starting value.
Why the unit matters
A share, such as the percentage of records with complete location information, can be expressed as a percentage. When that share changes, readers need to know whether a report is subtracting the shares or calculating a relative change.
The Open University's mathematics course explicitly distinguishes subtraction in percentage points from percentage increase or decrease. That distinction applies equally to a public-data quality measure or a published service share. OpenLearn: Percentage points.
Example
Hypothetical example: In one release, 80% of eligible call records have a usable location. In the next comparable release, 90% do. The improvement is 10 percentage points: 90% − 80%.
Relative to the original 80%, the increase is 12.5%: (90 − 80) ÷ 80 × 100. A statement saying only “location completeness improved by 10%” would be ambiguous and would not accurately describe either calculation without clarification.
Common confusion
Ten percentage points does not mean ten records. If both releases contain 1,000 eligible records, the example corresponds to 800 and 900 usable locations, a difference of 100. If their totals differ, the change in usable-record counts must be calculated separately.
Percentage points also differ from the units in a rate such as calls per 1,000 residents. A rate moving from 10 to 12 calls per 1,000 has increased by two calls per 1,000. Those are not percentage points unless the underlying measure is actually expressed as a percentage.
Keep the underlying shares comparable
State what each percentage uses as its numerator and denominator. If one release excludes records for which a location is not applicable and another includes them, subtracting the published percentages does not isolate a change in recording quality.
Use the same rounding convention. Subtracting rounded percentages can give a slightly different result from subtracting unrounded figures. Preserve the underlying values when available and label approximate results appropriately. A useful sentence reports both endpoints and the difference: “The share increased from 80% to 90%, a gain of 10 percentage points.”