Which wording distinction should a bid writer preserve?
The Office for National Statistics defines a percentage point as the difference between percentages and distinguishes that from a percentage change. Use clear wording in an engineering or EPC capability response so a reader can reproduce the claimed comparison.
Returnables Agency is independent and is not affiliated with the Office for National Statistics. The worked example below is invented. It is not evidence of this agency's results, a client outcome or a promised improvement.
Sources: Office for National Statistics: Percentages and percentage points
How do the two calculations differ?
Illustrative example. Two fictional comparable review batches each contain 100 eligible documents. The first has 80 accepted on the declared first-pass basis, while the second has 90. Both use the same acceptance definition and eligibility rule.
| Measure | Calculation | Meaning |
|---|---|---|
| First rate | 80 / 100 = 80 percent | Original rate |
| Second rate | 90 / 100 = 90 percent | Final rate |
| Percentage-point change | 90 - 80 = 10 percentage points | Difference between the rates |
| Relative change | (90 - 80) / 80 x 100 = 12.5 percent | Increase relative to the original rate |
| Unsupported wording | Improved by 10 percent | Ambiguous or incorrect for this comparison |
Which evidence must accompany a real claim?
Retain the numerator, denominator, period, task scope and acceptance definition for each rate. Check whether the sample mix, exclusions, rework treatment or measurement method changed. Equal percentages do not make unlike populations comparable, and correct arithmetic does not prove who caused the change.
Use only evidence the bidder is authorised to disclose. Link the draft sentence to its approved source and the factual reviewer. If a relevant comparison cannot be supported, remove the quantified claim or state the limited observation accurately. Do not invent missing historic counts to complete a case study.
How should difficult cases be written?
With an original rate of zero, a relative percentage increase calculated by dividing by that original rate is undefined. Keep the actual before and after rates rather than invent an infinite or convenient improvement. A percentage-point difference may still be calculable.
For falling rates, name the metric and direction explicitly: a lower defect rate is not the same claim as a higher acceptance rate unless the definitions support that relationship. Have the bid owner approve the final wording. No performance guarantee follows from a past measured result.
Common questions
What else should the bid team know?
Is an increase from 80 to 90 percent a 10 percent increase?
It is 10 percentage points, or a 12.5 percent relative increase from 80 percent.
Does correct arithmetic prove causation?
No. The evidence must support any claim about what caused the change.
Can a relative increase be calculated from a zero starting rate?
Not by the ordinary percentage-change formula. Preserve the before and after rates without inventing a relative result.
Primary sources
- Percentages and percentage pointsOffice for National Statistics