ECO 230
Marianna Kudlyak
Economic Statistics
Spring 2007
Answer Key to Homework #8
Total points: 10.
Exercise 1
. Newbold, Carlson and Thorne, Problem 6.75, p.227.
Exercise 2
. Newbold, Carlson and Thorne, Problem 6.91, p.228.
Exercise 3.
The amount of time before the first score by either team in college hockey games is
exponentially distributed with a mean of 8.7 minutes. Consider a sample of 100 games a
team played over the last years. Assuming that the games are independent, what is the
probability that in more than 20% of the games, the first period ends with neither team
scoring? A period in hockey lasts 20 minutes.
Probability that the first period ends without a goal: P(T>20), where T is an exponentially
distributed random variable that denotes time between the start of the first period and the
first goal. Hence,
λ
λ
20
20
)
1
(
1
)
20
(
1
)
20
(
−
−
=
−
−
=
<
−
=
>
e
e
T
P
T
P
For exponential distribution we have
10
.
0
)
20
(
7
.
8
1
7
.
8
1
)
(
7
.
8
20
=
=
>
=
=
=
−
e
T
P
T
E
λ
λ

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