1
Q
1.
(2.23) As a part of a classic experiment on mutations, ten aliquots of identical size were taken from the same
culture of the bacterium
E.coli
. For each aliquot, the number of bacteria resistant to a certain virus was
determined. The results were as follows:
14 15 13 21 14 14 26 16 20 13
The mean, median and the third quartile are, respectively, :
(a)
16.6; 14.5; 14
(b)
16.6; 14.5; 20
(c)
16.6; 14; 20
(d)
18.1; 14.5; 20
(e)
none of the preceding
Q
2.
Suppose a certain drug test is 99% sensitive and 99% speci±c, that is, the test will correctly identify a drug user
as testing positive 99% of the time, and will correctly identify a nonuser as testing negative 99% of the time.
Let’s assume a corporation decides to test its employees for opium use, and it is known 0.5% of the employees
use the drug. The probability that, given a positive drug test, an employee is actually a drug user, is:
(a)
0
.
3322
(b)
0
.
1622
(c)
0
.
99
(d)
0
.
01
(e)
none of the preceding
HINT: compare Question 1 in Assignment 3.
Solution to Q2:
Let
D
be the event of being a drug user and
N
indicate being a nonuser. Let
A
be the event of a positive
drug test. To compute:
P
(
D

A
). We know the following:
•
P
(
D
), or the probability that the employee is a drug user. This is 0.005.
•
P
(
N
), or the probability that the employee is not a drug user. This is 1

P
(
D
), or 0
.
995.
•
P
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 Fall '08
 Statistics, NHL, drug user

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