If the QC manager wants to estimate with 95 confidence the m

If the QC manager wants to estimate with 95% confidence, the mean life of light bulbs to within plus or minus 20 hours and also assumes the population standard deviation is 100 hours. How many bulbs need to be selected?

Solution

Note that      
      
n = z(alpha/2)^2 s^2 / E^2      
      
where      
      
alpha/2 = (1 - confidence level)/2 =    0.025  
      
Using a table/technology,      
      
z(alpha/2) =    1.959963985  
      
Also,      
      
s = sample standard deviation =    100  
E = margin of error =    20  
      
Thus,      
      
n =    96.03647052  
      
Rounding up,      
      
n =    97