1) The county health department just published a report saying that there is 1% chance that the deadly disease X exists in the population. There is a screening test for X, and you are encouraged to take the screening test. However, the test is not perfect. (No test is ever perfect). If you are sick, the test is positive in 90% of cases. If you are not sick, the test is positive in 4% of cases. a) Draw a tree diagram for the story above. Tests +ve) 0.9 Sick 0.01 0.1 Tests ve Population Tests +ve) 04. 0.99 Not Sick 0.6 Tests -ve b) You take a screen test. What is the probability that you have a positive result? 0.405 c) You take a screen test. What is the probability that you have a negative result? 0.595 d) Given that you have a negative result. What is the probability that you have the disease? e) Given that you have a positive result. What is the probability that you don't have the disease? f) Which scenario is more awful? d or e? Explain.

A First Course in Probability (10th Edition)
10th Edition
ISBN:9780134753119
Author:Sheldon Ross
Publisher:Sheldon Ross
Chapter1: Combinatorial Analysis
Section: Chapter Questions
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1) The county health department just published a report saying that there is 1% chance that the deadly
disease X exists in the population. There is a screening test for X, and you are encouraged to take the
screening test. However, the test is not perfect. (No test is ever perfect).
If you are sick, the test is positive in 90% of cases.
If you are not sick, the test is positive in 4% of cases.
a) Draw a tree diagram for the story above.
Tests +ve
0.9
Sick
0.01
0.1
Tests -ve
Population
Tests +ve)
0.99
0.4.
Not Sick
0.6
Tests -ve
b) You take a screen test. What is the probability that you have a positive result?
0.405
c) You take a screen test. What is the probability that you have a negative result?
0.595
d) Given that you have a negative result. What is the probability that you have the disease?
e) Given that you have a positive result. What is the probability that you don't have the disease?
f) Which scenario is more awful? d or e? Explain.
Transcribed Image Text:1) The county health department just published a report saying that there is 1% chance that the deadly disease X exists in the population. There is a screening test for X, and you are encouraged to take the screening test. However, the test is not perfect. (No test is ever perfect). If you are sick, the test is positive in 90% of cases. If you are not sick, the test is positive in 4% of cases. a) Draw a tree diagram for the story above. Tests +ve 0.9 Sick 0.01 0.1 Tests -ve Population Tests +ve) 0.99 0.4. Not Sick 0.6 Tests -ve b) You take a screen test. What is the probability that you have a positive result? 0.405 c) You take a screen test. What is the probability that you have a negative result? 0.595 d) Given that you have a negative result. What is the probability that you have the disease? e) Given that you have a positive result. What is the probability that you don't have the disease? f) Which scenario is more awful? d or e? Explain.
Expert Solution
Step 1

d) Given that the probability of having the disease is 1%=0.01

We have to find P(have disease | negative result) P(have disease | negative result)=P(have disease and negative result)P(negative result)=P(have disease and negative result)P(have disease and negative result)+P(do not have disease and negative result)=P(have disease)×P(negative result|have disease)P(have disease)×P(negative result|have disease)+P(do not have disease)×P(negative result|do not have disease)=0.01(1-0.90)0.01(1-0.90)+(1-0.01)(1-0.04)=0.00105108261509Answer(d): 0.00105108261509

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