Confidence Intervals
Example 1
● An urban economist wishes to estimate the mean
amount of time people spend traveling to work. He
obtains a random sample of 50 individuals who are in the
labor force and finds that the mean travel time is 24.2
minutes. Assuming the population standard deviation of
travel time is 18.5 minutes, construct and interpret a 95%
confidence interval for the mean travel time to work.
Interpretation: We estimate the mean travel time to work
to be between 19.1 and 29.3 minutes. (95% CI)
Fall 2008
1
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Example 2
●
The following data represent the battery life in hours,
for a random sample of 10 full charges on a fifth
generation iPod music player.
7.3
6.6
10.2
10.2
12.9
9.0
10.8
8.5
12.1
7.1
●
Construct a 98% confidence interval for the mean
number hours the battery will last on this player.
Assume
σ
= 1.5
.
Interpretation:
It is estimated that the mean number of
hours the battery will last on a fifthgeneration iPod
music player is between 8.37 and 10.57 hours. (98%CI)
Fall 2008
2
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 Spring '08
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 Statistics, Standard Deviation, Confidence Intervals Example

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