Statistics puzzle - answers by Friday

[RJT]

Here's an interesting little puzzle for everyone to have a hard think about.
I will post a solution to this on Friday, until then if you genuinely already know the answer, please keep it to yourself and let others have a go -

Suppose we have a diagnostic test for a particular disease which is 99% accurate.
A person is picked at random and tested for the disease.
The test gives a positive result. What is the chance that the person actually has the disease?

RJT.

[vonQuux]

Oh, what the hell. I'll be the guinea pig.

99%.

vQ

[Guynoire]

I think the puzzle is missing some information. Don't you need to know the disease percentage of the total population to answer this?

[MartyAce]

I've done this move too when I couldn't answer extra credit problems in class.

[Sonny]

If I may rewrite the example to make it a bit more topical (courtesy of Steve Forte):

Consider a software package that is 99% accurate at identifying card counters. Assuming that the casino sees 10,000 blackjack players and 1-out-of-100 of them are actually capable of counting cards, what are the chances of the software correctly identifying a card counter?

-Sonny-

[sagefr0g]

Originally Posted by Sonny

If I may rewrite the example to make it a bit more topical (courtesy of Steve Forte):

Consider a software package that is 99% accurate at identifying card counters. Assuming that the casino sees 10,000 blackjack players and 1-out-of-100 of them are actually capable of counting cards, what are the chances of the software correctly identifying a card counter?

-Sonny-

i'm just gonna guess for the fun of it.
it'll find 99 ploppies to be card counters and 1 card counter will be correctlly identified.
edit: erhh that's one out of a hundred a 0.01 probability.

[godeem23]

Originally Posted by Sonny

If I may rewrite the example to make it a bit more topical (courtesy of Steve Forte):

Consider a software package that is 99% accurate at identifying card counters. Assuming that the casino sees 10,000 blackjack players and 1-out-of-100 of them are actually capable of counting cards, what are the chances of the software correctly identifying a card counter?

-Sonny-

I think this needs to be reworded. The question can be interpreted in the following ways:
1. A person was chosen randomly and identified as a counter. What is the probability that they are actually a counter?
2. A person was random chosen and tested by the software package. What is the probability that the person was BOTH a counter AND identified as a counter?

[vonQuux]

Originally Posted by Guynoire

I think the puzzle is missing some information. Don't you need to know the disease percentage of the total population to answer this?

I was thinking the exact same thing.

vQ

[jack,jackson]

I keep coming up with -14

[bj bob]

My guess is.99²