STAT 408
Spring 2009
Version A
Name
ANSWERS
.
Section __________
Quiz 2
(10 points)
Be sure to show all your work; your partial credit might depend on it.
No credit will be given without supporting work.
1.
A statistics instructor teaches two statistics classes, one in the morning and one
in the afternoon.
Suppose the probability that the instructor gives a bad morning
lecture is 0.3.
If the morning lecture is bad, then the afternoon lecture is also bad
with probability 0.70.
However, if the morning lecture is good, then there is an
80% chance that the afternoon lecture will also be good.
M = {bad morning lecture},
A = {bad afternoon lecture}.
P( M ) = 0.30
(the probability that the instructor gives a bad morning
lecture is 0.3)
P( A

M ) = 0.70
(if the morning lecture is bad
, then the afternoon lecture
is also bad with probability 0.70)
P( A
'

M
'
) = 0.80
(if the morning lecture is good
, then there is an 80% chance
that the afternoon lecture will also be good)
a)
(4)
What proportion of the afternoon lectures are bad?
P( A )
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 Spring '08
 STEPANOV
 Statistics, The Instructor, Sunday School, 80%, morning lecture

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