SOLUTION: You plan to conduct a poll to estimate the percentage of consumers who are satisfied with long distance phone service. You want to be 90% confident that your sample percentage is w

Question 1152390: You plan to conduct a poll to estimate the percentage of consumers who are satisfied with long distance phone service. You want to be 90% confident that your sample percentage is within 2.5 percentage points of the true population value, and a Roper poll suggests that this percentage should be about 85%. How large must your sample be?
Answer by Boreal(15235) (Show Source): You can put this solution on YOUR website!
90% CI has z=1.645
SE is sqrt(p*(1-p)/n)
half-interval is 0.025
so 1.645*sqrt(0.85*0.15/n)=0.025
square everything
2.706*.1275/n=0.000625
so n=2.706*0.1275/0.000625=552.04 or 553 needed.
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