← A Number's True Size · Lesson 4 of 4
A test in beans
These numbers are made up to show the idea; they are not any real test. Picture 1,000 people. 10 of them have a condition. A test finds 9 of those 10, and also wrongly flags about 5 in every 100 of the people who do not have it.
Counting it out: 9 true alarms, and about 50 false ones from the 990 who are well (5% of 990 is about 50). So about 59 people test positive, and only 9 of them have the condition.
In percentages this feels like a puzzle. In counts it is a bowl of beans. People get this kind of question right far more often when it is shown as counts of people, not as percentages.
In the made-up example, of the people who test positive, about how many have the condition?
A lesson is kept once you've answered the question and written at least 8 words. Nobody grades it; it stays on this device.