Count the missing denominator

The idea

A percentage describes a part relative to a specified whole. Before treating a number as evidence, identify which events entered the count, which were excluded, and the interval covered. A numerator without its denominator can make an ordinary event appear exceptional.

Worked example

A neighborhood newsletter reports that bicycle thefts doubled, from four to eight. Meanwhile, the bicycle parking area expanded from 100 regularly parked bikes to 400. Using these rough exposure counts, thefts fell from four per hundred bikes to two per hundred. Neither statistic alone describes the entire problem: more victims exist, but estimated risk per parked bicycle is lower. The committee records both counts and improves its exposure measurement before deciding where to install secure racks.

Use it carefully

An exposure measure must match the question. Unique bicycles, parking hours, and riders are different denominators. Dividing by a convenient total does not make the resulting rate meaningful. Compare consistent definitions across periods, retain absolute counts, and ask whether reporting practices changed. A rate estimates a pattern in recorded data; it does not explain why that pattern occurred.

RETRIEVE. REASON. REMEMBER.

Knowledge check.

Choose one answer for each question, then check your reasoning.