In: Statistics and Probability
2. A manufacturing firm in Pittsburgh, Pennsylvania receives shipments of part from two suppliers (we’ll call them A1 and A2. The probability that the parts come from A1 is 0.65, the probability that they come from A2 is 0.35.
Past experience has shown that if the parts came from A1, the probability that they are good is 0.98. If the parts came from A2, the probability that they are good is 0.95.
a. Draw the probability tree
b. What is the probability that a part came from A1 and is good?
c. What is the probability that a part came from A2 and is bad?
d. Using the information given and your results from b and c complete the contingency table below
Supplier A1 |
Supplier A2 |
||
Good part |
|||
Bad part |
|||
e. What is the probability that a randomly selected part is bad?
f. A randomly selected part has been found to be bad. What is the probability that is from Supplier A1?
g. A randomly selected part has been found to be bad. hat is the probability that is from Supplier A2?
Solution:
Given:
P(A1) = 0.65
P(A2) = 0.35
if the parts came from A1, the probability that they are good is 0.98.
That is: P(Good | A1) = 0.98
If the parts came from A2, the probability that they are good is 0.95.
P( Good | A2) = 0.95
Part a. Draw the probability tree
Part b. What is the probability that a part came from A1 and is good?
P(A1 and Good) = P( Good | A1) x P(A1)
P(A1 and Good) = 0.98 x 0.65
P(A1 and Good) = 0.637
Part c. What is the probability that a part came from A2 and is bad?
P( A2 and Bad) = P( Bad | A2 ) x P(A2)
P( A2 and Bad) = 0.05 x 0.35
P( A2 and Bad) = 0.0175
Part d. Using the information given and your results from b and c complete the contingency table below
Supplier A1 | Supplier A2 | ||
---|---|---|---|
Good part | 0.637 | 0.3325 | P(Good) = 0.9695 |
Bad part | 0.013 | 0.0175 | P(Bad) = 0.0305 |
0.65 | 0.35 | 1 |
Part e. What is the probability that a randomly selected part is bad?
P(Bad) =0.013 + 0.0175
P(Bad) = 0.0305
Part f. A randomly selected part has been found to be bad. What is the probability that is from Supplier A1?
P(A1 | Bad) =............?
From table , we have:
P(A1 and Bad) = 0.013
and P(Bad) = 0.0305
Thus
Part g. A randomly selected part has been found to be bad. What is the probability that is from Supplier A2?
P(A2 | Bad) = .........?