4. Provided in homework 5.xlsx is a set of restoration times (in hours) for a particular kind of failed power transformer from a power utility. The data were collected for recent transformer failures by a fairly novice work crew. Calculate a 95% confidence interval for the standard deviation of restoration time. a. Find a 95% confidence interval for the mean restoration time. Interpret. b. Find a 99% confidence interval for the standard deviation of transformer restoration time. Interpret. c. Power utilities, as they tend to be monopolies, often share best practices with each other as they don’t really compete for each other’s business. A quality assurance metric that many utilities use is the proportion of transformers taking longer than 135 minutes to restore. Find a 90% confidence interval to estimate this proportion.
4. Provided in homework 5.xlsx is a set of restoration times (in hours) for a particular kind of failed
power transformer from a power utility. The data were collected for recent transformer failures
by a fairly novice work crew. Calculate a 95% confidence interval for the standard deviation of
restoration time.
a. Find a 95% confidence interval for the mean restoration time. Interpret.
b. Find a 99% confidence interval for the standard deviation of transformer restoration
time. Interpret.
c. Power utilities, as they tend to be monopolies, often share best practices with each
other as they don’t really compete for each other’s business. A quality assurance metric
that many utilities use is the proportion of transformers taking longer than 135 minutes
to restore. Find a 90% confidence

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