Question

A
pollster wants to construct a 95% confidence interval for the
proportion of adults who believe that economic conditions are
getting better. A Gallup poll taken in July 2010 estimates this
proportion to be 0.33. Using this estimate, what sample size is
needed so that the confidence interval will have a margin of error
of 0.034 ?

Write only an integer as your answer .

Answer #1

Minimum sample size for CI level = 95% and Desired Margin of Error, E=0.034, is given by:

note:

Best estimate is

(from z table)

Since the minimum n has to be integer, we take the ceiling of above number and get n = 735

Note that if we take exact z-value, we will get a same answer as the exact calculation will be:

This will round up to: n=735

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