Credit Union Incomes. Find the 90% confidence interval of the population mean for the incomes of Pennsylvania credit unions. A random sample of 50 credit unions is shown. The data are in thousands of dollars.

**Solution: **We have the following table:

The population standard deviation is not known, but the sample size is large enough to apply a normal approximation using_{}as an approximation of_{}. Therefore, we get that the_{} interval is approximated as

_{}

where_{}corresponds to the two-tailed z-critical value for_{}. Hence, we get

_{}

This means that there is a probability of_{} that the actual population mean_{} is contained by the interval_{}.

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