00:01
We have some tests for marijuana, and let's look at the information.
00:06
We have 143 positive tests, but out of these, 244 are false positive.
00:15
So we have 143 positive and 157 negative tests.
00:19
And we're going to see which of these are true positives and which are false.
00:24
So 24 are false, which means the remainder are going to be true positives, as in they actually took cannabis and tested positive.
00:36
So 119 are true.
00:39
Similarly, at the 157 negative tests, we have three false negatives, so they actually took cannabis but tested negative anyway, and 154 are true negatives.
00:54
What's the probability? a randomly selected subject did not use marijuana.
01:00
So i'm going to make a fraction here.
01:02
On the denominator, i'm going to have all subjects.
01:08
On the numerator, i'm going to have the ones who meet the criteria, ones who did not use it.
01:19
And that's going to work because each subject has an equal chance of being chosen here.
01:24
We're randomly selecting one.
01:26
So if i just add up, i can just add these up and i'll know how many tests were done and then i'll know how many subjects there were...