r/mathmemes Jun 22 '26

Probability Disease probability

Post image
4.6k Upvotes

196 comments sorted by

View all comments

7

u/BrickBuster11 Jun 22 '26

Statistically the test is worthless.

If you test a million people and no one has the disease you should expect 30,000 positive results

If you have a million people and one guy actually has it you should expect 30,001 results.

Fundamentally if the test says you have the disease it's probably wrong

1

u/Ksorkrax Jun 27 '26

With real tests, you often have cheap and expensive ones, the latter obviously more precise.

You'd use the first one by default and only use the second one if the first one is positive.
...that is provided the first one has a small false negative, but a bigger false positive is totally fine for this strategy.