# Homework 9 _SOLUTION

1. Walpole 8.402. Walpole 8.44 3. Walpole 8.48 4. Walpole 9.2. Note: When one or two-sided is not speciﬁed, ﬁnd a two-sided CI. 5. Walpole 9.6 6. Your company manufactures electronic sensors. Your job is to determine the lifetime (in days, if powered continuously) which will be used in sales material for the sensor. A sample of 36 sensors is found to have an average lifetime of 352 days. (a) It is known that the standard deviation of the lifetime is 20.0 days. Find a 95% conﬁdence interval for the population mean lifetime. (b) Now, the population standard deviation is not known, but the sample standard deviation (from the same sample of 36) is 20.0 days. Find a 95% conﬁdence interval for the population mean lifetime. (c) Would increasing the sample size tend to make the conﬁdence interval wider or narrower? (d) Would a 99% conﬁdence interval be wider or narrower than the 95% conﬁdence interval?

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