7.118 Insulation study. A manufacturer of electric motors tests insulation at a high temperature (250̊ C) and records the number of hours until the insulation fails.⁴⁴ The data for five specimens are

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The small sample size makes judgment from the data difficult, but engineering experience suggests that the logarithm of the failure time will have a Normal distribution. Take the logarithms of the five observations and use t procedures to give a 90% confidence interval for the mean of the log failure time for insulation of this type.