The proper operation of typical home appliances requires voltage levels that do not vary much. Listed below are 20 voltage levels (in volts) at a random house on 20 different days.

The mean of the data set is 120; the sample standard deviation is 0.18; if the normality plot is not provided you may assume that the voltages are normally distributed.

Construct a 90% confidence interval for the variance (standard deviation) of all voltages in the house.

1. Procedure: Select an answer One variance χ² procedure One mean T procedure One proportion Z procedure One mean Z procedure
2. Assumptions: (select everything that applies)
   • Normal population
   • Simple random sample
   • The number of positive and negative responses are both greater than 10
   • Population standard deviation is known
   • Sample size is greater than 30
   • Population standard deviation is unknown
3. Unknown parameter: Select an answer p, population proportion σ², population variance μ, population mean  
4. Point estimate: Select an answer sample proportion, p̂ sample mean, x̄ sample variance, s² = (Round the answer to 3 decimal places)
5. Confidence level % and $\displaystyle \alpha=$ , also
   •  $\displaystyle \frac{\alpha}{{2}}=$ , and $\displaystyle {1}-\frac{\alpha}{{2}}=$
   • Critical values: (Round the answer to 3 decimal places)
     • left= right=
6. Margin of error (if applicable): (Round the answer to 3 decimal places)
7. Lower bound: (Round the answer to 3 decimal places)
8. Upper bound: (Round the answer to 3 decimal places)
9. Confidence interval:(, )
10. Interpretation: We are % confident that the true population variance is between and .