statistics
posted by Tom .
In a poll of 700 selected people in the USA who are 50 years or older, 20% said they would prefer a complete overhauling of the social security system. Estimate the population proportion of the USA of 50 years or older who would prefer a complete overhauling of the social security system. Use a 95% confidence level. What is the minimum number of people required that would give a sampling error of only 5%?

Try this formula:
n = [(zvalue)^2 * p * q]/E^2
= [(1.96)^2 * .2 * .8]/.05^2
I'll let you finish the calculation.
Note: n = sample size needed; .2 (20%)for p and .8 (which is 1  p) for q. E = maximum error, which is .05 (5%) in the problem. Zvalue is found using a ztable (for 95%, the value is 1.96). Symbols: * means to multiply and ^2 means squared.
I hope this will help get you started.
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