Answer:
There is a 28.57% probability that a randomly selected participant who died by the end of the study was a non-smoker.
Step-by-step explanation:
We have the following probabilities:
A 15% probability that a participant is classified as a heavy smoker.
A 25% probability that a participant is classified as a light smoker.
A 100% - 25% - 15% = 60% probability that a participant is classified as a non smoker.
A x% probability that a non smoker dies.
A 3x% probability that a light smoker dies.
A 5x% probability that a heavy smoker dies.
This can be formulated as the following problem:
What is the probability of B happening, knowing that A has happened.
It can be calculated by the following formula
![P = \frac{P(B).P(A/B)}{P(A)}](https://tex.z-dn.net/?f=P%20%3D%20%5Cfrac%7BP%28B%29.P%28A%2FB%29%7D%7BP%28A%29%7D)
Where P(B) is the probability of B happening, P(A/B) is the probability of A happening knowing that B happened and P(A) is the probability of A happening.
This problem is:
What is the probability of the participant being a non-smoker, given that he died?
P(B) is the probability that the participant is a non smoker. So
![P(B) = 0.6](https://tex.z-dn.net/?f=P%28B%29%20%3D%200.6)
P(A/B) is the probability that the participant dies, given that he is a non smoker. So:
![P(A/B) = x](https://tex.z-dn.net/?f=P%28A%2FB%29%20%3D%20x)
P(A) is the probability that the participant dies:
![P(A) = P_{1} + P_{2} + P_{3}](https://tex.z-dn.net/?f=P%28A%29%20%3D%20P_%7B1%7D%20%2B%20P_%7B2%7D%20%2B%20P_%7B3%7D)
is the probability that a heavy smoker is selected and that he dies. So:
![P_{1} = 0.15*5x = 0.75x](https://tex.z-dn.net/?f=P_%7B1%7D%20%3D%200.15%2A5x%20%3D%200.75x)
is the probability that a light smoker is selected and that he dies. So:
![P_{2} = 0.25*3x = 0.75x](https://tex.z-dn.net/?f=P_%7B2%7D%20%3D%200.25%2A3x%20%3D%200.75x)
is the probability that a non-smoker is selected and that he dies. So:
![P_{3} = 0.60*x = 0.60x](https://tex.z-dn.net/?f=P_%7B3%7D%20%3D%200.60%2Ax%20%3D%200.60x)
The probability that a participant dies is:
![P(A) = P_{1} + P_{2} + P_{3} = 0.75x + 0.75x + 0.60x = 2.10x](https://tex.z-dn.net/?f=P%28A%29%20%3D%20P_%7B1%7D%20%2B%20P_%7B2%7D%20%2B%20P_%7B3%7D%20%3D%200.75x%20%2B%200.75x%20%2B%200.60x%20%3D%202.10x)
The probability of the participant being a non-smoker, given that he died, is:
![P = \frac{P(B).P(A/B)}{P(A)} = \frac{0.6x}{2.10x} = \frac{0.6}{2.10} = 0.2857](https://tex.z-dn.net/?f=P%20%3D%20%5Cfrac%7BP%28B%29.P%28A%2FB%29%7D%7BP%28A%29%7D%20%3D%20%5Cfrac%7B0.6x%7D%7B2.10x%7D%20%3D%20%5Cfrac%7B0.6%7D%7B2.10%7D%20%3D%200.2857)
There is a 28.57% probability that a randomly selected participant who died by the end of the study was a non-smoker.