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# calling dr killdare

Wed 25 Dec, 2013 04:26 am
One in a Thousand people have a rare disease. But fortunately there is a test that is for 99% accurate. What is the chance that you are sick if the test says you are?

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Type: Question • Score: 0 • Views: 1,035 • Replies: 7
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contrex

1
Wed 25 Dec, 2013 04:41 am
If the test is 99% accurate, and yours came back positive, then there is a 99% chance that you have the disease.
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Jack of Hearts

1
Wed 25 Dec, 2013 01:18 pm
@whimsical,
If the test is to show you are sick, (positive = sick), it's a 99% chance you are sick.
If the test is to show you are healthy, (positive = healthy), it's a 50-50 chance you're sick.
(If the test is inconclusive, there's a 98% - 99% chance you are sick.)
Jack of Hearts

1
Wed 25 Dec, 2013 01:48 pm
@Jack of Hearts,
Note: I have obviously assumed parameters that may not be the case, e.g. all but 99% are healthy, or sick; and that 1% is inaccurate and just not inconclusive, if even only in part.
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markr

1
Wed 25 Dec, 2013 09:59 pm
@whimsical,
11/122
whimsical

1
Fri 27 Dec, 2013 01:34 am
@markr,
Is that about 9% Mark? How did you arrive at that? It's so differrent from whar some of the others say.

my feeling was that it be 50%
contrex

1
Fri 27 Dec, 2013 02:29 am
@whimsical,
whimsical wrote:

Is that about 9% Mark? How did you arrive at that? It's so differrent from whar some of the others say.

my feeling was that it be 50%

When you say "the test is 99% accurate", do you mean that for every 100 people with the disease who are tested, there are 99 positive results? Or do you mean that for every 100 people without the disease, there is one positive result? Or what?
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markr

1
Fri 27 Dec, 2013 02:48 am
@whimsical,
Yes, about 9%. Assume a population of 100,000. 100 are sick.
Code:``` pos neg --- --- sick 99 1 not sick 999 98901 ```

So, if you test positive, the probability is 99/(99+999) that you are sick.
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