The director of quality at a light-bulb factory needs to estimate the average li
ID: 3181188 • Letter: T
Question
The director of quality at a light-bulb factory needs to estimate the average life of a large shipment of light bulbs. The process standard deviation is known to be 180 hours. A random sample of 81 light bulbs indicated a sample average life of 400 hours.
Set up a 95% confidence interval (CI) estimate of the true average life of light bulbs in this shipment.
a1. The critical value is ...............................;
a2. The lower limit of CI is ..........................., and the upper limit of CI is ..............................;
Based on your results given in part a, do you think that the manufacturer has the right to state that the light bulbs last an average of 450 hours? ................................. (enter Y for "yes", N for "No")
Explanation / Answer
a) critical value for 95% CI z=1.96
a2) std error =std deviation/(n)1/2 =20
lower limit =mean -z*std errror =360.80
upper limit =mean +z*std error =439.20
No as our interval does not contain that as probable value