The capacities (in ampere-hours) were measured for a sample of 120 batteries. Th

Question

The capacities (in ampere-hours) were measured for a sample of 120 batteries. The average was 178, and the standard deviation was 14. a) Find a 99% confidence interval for the mean capacity of batteries produced by this method b) An engineer claims that the mean capacity is between 176 and 180 ampere-hours. With what level of confidence can this statement be made? c) Approximately how many batteries must be sampled so that a 99% confidence interval will specify the mean to within plusminus 2 ampere-hours?

Explanation / Answer

a) n=120

mean=178

sd=14

a) Ci = mean - sd/sqrt(n) * t(0.005,119) , mean + sd/sqrt(n) * t(0.005,119) =

178-(14/sqrt(120))*2.618 , 178+(14/sqrt(120))*2.618

=174.65 , 181.34

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