Collection
Make the numbers answer the right question
Check percentages, averages, units and uncertainty before a neat calculation turns into a misleading decision.
15 elements · Explore in any order.
1 checklist · 4 concepts · 3 heuristics · 5 protocols · 2 questions
15 elements
Ask what the percentage is a percentage of
A percentage without its whole is an unfinished sentence.
Read the idea ConceptSeparate percentage points from percentage change
Two extra points and twenty percent more can describe the same movement.
Read the idea ProtocolMultiply successive percentage changes
The second change usually acts on what the first change left behind.
Read the idea ProtocolWeight a combined average by what it represents
A tiny group does not acquire equal weight because its result occupies one spreadsheet cell.
Read the idea HeuristicChoose the average that fits the question
The arithmetic mean can be correct and still describe nobody's ordinary day.
Read the idea ConceptRead a percentile as a rank, not a score
The 90th percentile is a position in a crowd, not ninety correct answers out of a hundred.
Read the idea QuestionPut variation beside the average
Five minutes on average can mean five minutes every time—or an unpleasant lottery.
Read the idea ProtocolBuild an overall rate from totals
Averaging the printed rates may average the wrong things.
Read the idea ProtocolLet the units check the formula
Units are a second line of reasoning, not decoration after the answer.
Read the idea HeuristicRound for reporting, not by accident during calculation
Rounding errors can accumulate while each individual cell looks harmless.
Read the idea ConceptSeparate repeatable measurements from correct measurements
A clock can be reliably five minutes wrong.
Read the idea ChecklistKeep a proportion's value separate from its display
The symbol can disappear while the factor of a hundred remains.
Read the idea HeuristicSet a rough bound before trusting the exact answer
An extra zero is less convincing when you already know the right neighborhood.
Read the idea ProtocolSubtract the overlap before combining counts
Two category totals do not necessarily describe two different crowds.
Read the idea ConceptRead expected value as a weighted average of possibilities
An expected outcome need not be an outcome that ever occurs once.
Read the idea