More Probabilities ...

sunny-dave | 21:21 Wed 29th Oct 2014 | ChatterBank

68 Answers

... but very practical ones this time.

You are invited to go for a screening test for an illness - you have no symptoms.

About 1% of the screened group will have the illness.

The test is 99% accurate at diagnosing people who have the illness.

You get a 'positive' result.

How likely is it that you actually have the illness?

99%?

That's what many people think.

Wouldn't You Spell A Rat If Invited To Be In The Audience Of...

What Do You Make Of This 'cos I'm Flumoxed.

Answers

1 to 20 of 68

1 2 3 4 Next Last

Best Answer

No best answer has yet been selected by sunny-dave. Once a best answer has been selected, it will be shown here.

For more on marking an answer as the "Best Answer", please visit our FAQ.

sunny-dave

Question Author

The true answer is "You just don't know" - until they share something that they will be very coy about ... which is the percentage rate of "false positives".

Say they eventually admit that this is about 2% of people scanned (not a bad rate actually).

This then means that (if you get a positive result) your actual chance of having the illness is ... about 33%. Not the 99% that they waved at you in the first place.

[ explanation :

scan 1,000,000 people

10,000 will have the illness and 9,900 will be given a (true) positive diagnosis

990,000 will not have the illness - 19,800 will be given a (false) positive diagnosis

you have an 1 in 3 chance of being in the 'true' group ]