BASE RATES

why a “yes” from a test that is right 99 times in 100 is wrong more than nine times in ten when one in a thousand has the thing

Built as a memo rather than a table: the thing worth hanging it up for reads across the room, with the full reference in smaller type below it.

Three sizes in one archive — from a sheet on the fridge to a poster on the wall.

Formats: A4 · A3 · A2
What you get: Print-ready PDFs, one per size, with every letterform converted to outlines — nothing to install, nothing to shift.
Subject: maths · everyone · memo · study

Read it as: of everyone the test told “yes”, this share really has the thing. Every figure is arithmetic from the conditions stated on the sheet itself, not a claim about any particular test.

THE GROUP WEIGHS MORE THAN THE TEST

What changes is the group, not the test. The same test — 99 % and 99 % — on people who came in with complaints, where one in ten has it: 108 are told “yes”, and 99 of them really have it. That is 91.7 % instead of 9.0 %, with the test identical to the last digit. Only the share of people walking through the door is different.

The other way round: make the test ten times more careful with those who are clear — 99.9 % instead of 99 % — but leave the thing as rare as 1 in 1,000. Then 1,989 in a million are told “yes”, and 990 of them have it: 49.8 %. The test got ten times better, and the answer is still a coin toss, not certainty.

On the sheet

Read it as: of everyone the test told “yes”, this share really has the thing. Every figure is arithmetic from the conditions stated on the sheet itself, not a claim about any particular test.
Colour carries meaning, but meaning never rests on colour alone — the sheet reads in black and white.
Nothing about how it was made: the sheet carries only what the reader needs.

Printing

Three sizes in the archive: A4 — 210×297 mm, A3 — 297×420 mm, A2 — 420×594 mm.
All text is converted to outlines. No fonts to install, and nothing shifts on another machine.
White background and sparing fills — it prints on a home printer without draining the cartridge.
Margins of at least 10 mm: a home printer cannot print to the edge.
No stroke thinner than 0.3 mm, so nothing disappears in print.

Why the numbers can be trusted. Every countable number here is computed from data rather than typed in, and the sheet must pass 13 independent checks before it is allowed to build — how many items there are, the order they come in, and the claim in the highlighted box itself. If one fails, there is no file.

BASE RATES

THE GROUP WEIGHS MORE THAN THE TEST

On the sheet

Printing

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