Statistical Illusions Entry #0426 Classified Declassified

Why an average can describe nobody in the room

The average income in a room of ten people can be a number that none of them earn, and yet it is reported as if it describes them all.

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Plate 67 — a summary statistic that summarised nothing in the room.

Intuition test — answer before you read on

Nine workers earn $30k and their manager earns $300k. The average is $57k. How many people in the room earn the average?

Nine workers earn thirty thousand dollars a year. Their manager earns three hundred thousand. The average income in the room is fifty-seven thousand — a figure that describes none of the ten people present. A journalist reports the average, and the reader imagines a room of moderately comfortable professionals. The room contains nothing of the sort.

What everyone sees

The average is received as a representative number. It is expected to describe the typical case, and in symmetric distributions it does. But the expectation survives even when the distribution is skewed, because the word average carries an implicit promise of centrality that the arithmetic does not guarantee. The reader has no reason to suspect the promise is broken unless the distribution is shown, and distributions are rarely shown.

What is actually happening

The arithmetic mean is sensitive to outliers in a way that the median is not, and skewed distributions — income, wealth, city populations, company sizes — are the norm, not the exception. Reporting the mean of a skewed distribution systematically overstates the central tendency, because the tail pulls the average away from where most observations cluster. Huff described this as one of the oldest statistical misrepresentations: choosing the measure of central tendency that tells the story you want.

Why it stays hidden

The distortion hides behind the familiarity of the word. Average is the first statistical concept most people learn, and its intuitive meaning — normal, typical, representative — is so deeply embedded that questioning it feels pedantic. The mathematical definition (sum divided by count) and the colloquial meaning (what most people experience) are different things, but the single word covers both, and the listener defaults to whichever meaning is more convenient.

Central tendency erases the spread. The average describes the arithmetic, not the room.

Central tendency erases the spread. The average describes the arithmetic, not the room.

The hidden part — entry #0426

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Central tendency erases the spread. The average describes the arithmetic, not the room.

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Sources & further reading 3
  1. Huff, "How to Lie with Statistics", 1954
  2. Wheelan, "Naked Statistics: Stripping the Dread from the Data", 2013
  3. Spiegelhalter, "The Art of Statistics: Learning from Data", 2019

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