# Math 5090–001, Fall 2009 Solutions to Assignment 3

Math 5090–001, Fall 2009
Solutions to Assignment 3
Chapter 12, Problem 1. (a) We want to find a and b such that
P {X̄ ≤ a | &micro; = 20} = P {X̄ ≥ b | &micro; = 20} = 0.05.
If &micro; = 20, then X̄ ∼ N(20 , σ 2 = 1/16). Therefore,
0.05 = P {X̄ ≤ a | &micro; = 20} = P
Therefore,
a − 20
≈ −1.645
1/4
⇒
a − 20
N(0 , 1) ≤
1/4
.
a ≈ 19.58875.
Similarly,
0.05 = P {X̄ ≥ b | &micro; = 20} = P
b − 20
N(0 , 1) ≥
1/4
.
Therefore,
b − 20
≈ 1.645
1/4
⇒
b ≈ 20.41125.
(b) We are asked to find β = P {not reject | &micro; = 21} in both cases. For
the critical region A:
β = P {X̄ ≥ 19.58875 | &micro; = 21} = P
19.58875 − 21
N(0 , 1) ≥
1/4
≈ 1.
This is awful. And for the region B:
β = P {X̄ ≤ 20.41125 | &micro; = 21} = P
20.41125 − 21
N(0 , 1) ≤
1/4
≈ 0.01.
Excellent!
(c) We are asked to find β = P {not reject | &micro; = 19} in both cases. For
1
the critical region A:
β = P {X̄ ≥ 19.58875 | &micro; = 19} = P
19.58875 − 19
N(0 , 1) ≥
1/4
≈ 0.01.
This is excellent. And for the region B:
β = P {X̄ ≤ 20.41125 | &micro; = 19} = P
20.41125 − 19
N(0 , 1) ≤
1/4
≈ 1.
This is awful!
(d) We are asked to compute
α = 1 − P {19.58875 ≤ X̄ ≤ 20.41125 | &micro; = 20}
19.58875 − 20
20.41125 − 20
=1−P
≤ N(0 , 1) ≤
1/4
1/4
= 1 − P {−1.645 ≤ N(0 , 1) ≤ 1.645}
≈ 0.1.
(e) We are asked to find two probabilities: First,
P {19.58875 ≤ X̄ ≤ 20.41125 | &micro; = 19} = P
19.58875 − 19
20.41125 − 19
≤ N(0 , 1) ≤
1/4
1/4
= P {2.355 ≤ N(0 , 1) ≤ 5.645}
≈ 0.003;
and then
P {19.58875 ≤ X̄ ≤ 20.41125 | &micro; = 21} = P
19.58875 − 21
20.41125 − 21
≤ N(0 , 1) ≤
1/4
1/4
= P {−5.645 ≤ N(0 , 1) ≤ −2.355}
≈ 0.003.
That is,
P not reject |&micro; − 20| = 1 ≈ 0.003.
2
Chapter 12, Problem 2. (a) We want
α = P {reject | θ = 2} = P {both the same color | θ = 2}
= P {both white | θ = 2} + P {both black | θ = 2}
1 1
1
= + = .
4 4
2
[This isn’t a very good test, is it?]
(b) There are 4 situations: θ = 0, 1, 3, 4.
P {not reject | θ = 0} = 0
P {not reject | θ = 1} = P {W1 ∩ B2 } + P {B1 ∩ W2 }
3 1
3
1 3
&times;
+
&times;
= .
=
4 4
4 4
8
3
P {not reject | θ = 3} = .
8
P {not reject | θ = 4} = 0.
(c) For α we want
α = P {reject | θ = 2} = P {both white | θ = 2} + P {both black | θ = 2}
1 1
1 1
1
=
&times;
+
&times;
= .
2 3
2 3
3
For β there are 4 situations: θ = 0, 1, 3, 4.
P {not reject | θ = 0} = 0
3 1
1
1
&times;1 +
&times;
=
P {not reject | θ = 1} =
4
4 3
2
1
P {not reject | θ = 3} =
2
P {not reject | θ = 4} = 0.
3
Chapter 12, Problem 6. (a) We want to find c such that
α = P {X1:n ≥ c | η = η0 }
= P {X1 ≥ c , . . . , Xn ≥ c | η = η0 }
= P {X1 ≥ c | η = η0 } &times; &middot; &middot; &middot; &times; P {Xn ≥ c | η = η0 }
= e−n(c−η0 )
Solve to get
1
c = η0 + ln
n
1
.
α
(b) We are asked to compute the following for all η1 &gt; η0 :
P {X1:n ≤ c | η = η1 } = 1 − P {X1:n ≥ c |η = η1 }
= 1 − e−n(c−η1 ) .
But c solves e−n(c−η0 ) = α. Therefore,
P {X1:n ≤ c | η = η1 } = 1 − e−n(c−η0 ) e−n(η1 −η0 )
= 1 − αe−n(η1 −η0 ) .
(c) We know from (b) that if η = η1 , then
β = αe−n(η1 −η0 )
⇒
n=
ln(α/β)
.
η1 − η0
But this is typically not an integer. Therefore, we choose conservatively; i.e.,
ln(α/β)
,
n≥1+
η1 − η0
where b&middot; &middot; &middot; c denotes the lowest-integer part function.
4
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