Identify the quantity before calculating
The UK Office for National Statistics distinguishes differences between percentages from relative percentage changes. A percentage point is the arithmetic gap between two percentage values. A relative change divides that gap by the original value. The label matters because a small-looking number can describe a substantial proportional movement when the starting rate is low.
For the example, subtract 20 from 25 to get five percentage points. Divide five by 20 and multiply by 100 to get a 25% relative increase. Neither calculation tells you how many people were involved without the underlying population or sample.
Keep the base and period visible
Pause the graphic long enough to read its title, footnote and dates. Does the percentage describe all residents, eligible voters, survey respondents or people who answered a particular question? A change in the group measured can make a simple before-and-after calculation misleading. Check whether the programme identifies comparable periods and definitions.
Also distinguish a monthly change from a change over the preceding year. Two presenters can cite different correct figures because they are describing different intervals. Write the start and end dates beside the number before deciding there is a contradiction.
Do not confuse a count with a share
If ten out of fifty items meet a condition, the share is 20%. If twenty out of one hundred meet it later, the share remains 20% while the count doubles. This invented example shows why a report about the number affected can differ from a report about the proportion affected. Ask which question the presenter is answering.
A graphic’s vertical axis can also begin above zero. Read its numerical scale instead of judging the size of a change solely from the height of two bars.
Share a complete sentence
Use wording that includes the measure, population, period and unit. For example: in this hypothetical sample, the share moved from 20% to 25%, a five-percentage-point rise. If you add the relative figure, label it explicitly. Preserve uncertainty notes supplied by the source, and follow the linked statistical release when a clipped television graphic omits essential definitions.
Related questions
Are five percentage points and 5% the same?
Not generally. The former is an arithmetic difference between rates; the latter may describe a relative change.
Can I calculate a percentage change from zero?
The usual relative-change calculation divides by the starting value, so a zero starting value needs different wording.
Sources and scope
Scope notes distinguish published documentation, supplied information and editorial methods. They do not report a hands-on service test.
- ONS: percentages — Primary statistical communication guidance distinguishes percentage points from percentage changes; calculations here are original hypothetical examples.
