00:01
All right, and your question, you're told a rare, but serious disease has been found in 1 % of the population.
00:06
And then we have this test that's going to be used to try to determine whether you have it or not.
00:12
I've decided to organize this information into a tree diagram, and you can see i already have my disease and no disease branches laid out.
00:21
The population that has the disease, they said it was 1 % of the population, so that would be 0 .01.
00:27
That means 99 % do not have the disease.
00:31
Now, the second set of branches represents the test and its effectiveness.
00:36
It will show up positive 98 % of the time for someone that actually has the disease.
00:42
So i put it on the branch that had disease.
00:45
That means it'll show up negative 2 % of the time for people that have the disease.
00:51
Now, the test will only show up positive 3 % of the time if the person does not actually have the disease, which means it'll show up negative 97 %.
01:02
Of the time.
01:04
Your question is, what is the probability that a person does not have the disease given that they tested positive? this is what's called a conditional probability, and you want to do the probability of no disease and positive as your numerator, and in the denominator, we want the probability of just being positive.
01:37
Okay, so the probability of no disease and positive, would be on this set of branches that i just circled.
01:44
That's no disease and positive...