Question

1) The probability of getting a certain disease is 0.2. A certain test is available which returns a positive result 90% of the time for those with the disease, and a negative result 75% of the time for those without the disease. Compute Pr{Test positive}. (Hint: It may be helpful to draw a probability tree, and round to two decimal place)


2) The probability of getting a certain disease is 0.2. A certain test is available which returns a positive result 90% of the time for those with the disease, and a negative result 75% of the time for those without the disease. Compute Pr{Test negative}. (Hint: It may be helpful to draw a probability tree, and round to two decimal places)

3) The probability of getting a certain disease is 0.2. A certain test is available which returns a positive result 90% of the time for those with the disease, and a negative result 75% of the time for those without the disease. Compute Pr{Test positive | No disease}. (Hint: It may be helpful to draw a probability tree, and round to two decimal places)


Please show work so I can do the rest!

Thank you!

Answer 1

I've got .39 for test positive, .61 for the negative test and .61 for the test positive/no disease but they were all marked as wrong so I was wondering if someone could help me with these questions? I used probability tree to find the right answers but it's clear that I think I am missing out something.

Thank you so much!

Answer 2

OK!!!

Test Positive = .38
Test Negative = .62
Test Positive/no disease = ????

I've got the first two right but the 3rd one is still messing me up!

Answer 3

0.2/0.8=0.25 for test positive/nodisease