Key takeaways
A percentage without its base is a number without a meaning. Ask "percent of what?" and most alarming statistics shrink to their real size.
- Always ask what the percentage is a percentage of.
- Relative risk sounds bigger than absolute risk for the same facts.
- Small bases make big percentages cheap.
Percent of what?
"Sales up 200 %" means little if last month's sales were three units. The base turns a percentage into a quantity.
Relative and absolute
A risk rising from 1 in 1,000 to 2 in 1,000 is a 100 % relative increase and a 0.1 percentage-point absolute increase. Headlines prefer the first; decisions need the second.
Three questions
- What is the base?
- Is this a change in a rate (points) or in a quantity (percent)?
- Over what period?
A number with no denominator is an opinion wearing a suit.
Related questions
- What is the difference between relative and absolute risk?
