Question

The standard deviation of the running time of a sample of 200 fuses was found to be 110 hours. Find a 99% confidence interval for the standard deviation of all such fuses. 

Answer 1

Degrees of freedom df=n-1=200-1=199.

$99\%CI=(s\sqrt{\frac{n-1}{\chi^2_{0.005}}},s\sqrt{\frac{n-1}{\chi^2_{0.995}}})=$

$=(110\sqrt{\frac{200-1}{254.14}},110\sqrt{\frac{200-1}{151.37}})=(97.34,126.12).$