What is the 95% confidence level?

Question

"A polling agency states that candidate Y currently has 40% of the vote in the upconing election. The agency used a random sample of 500 people in the US generated by computer. What is the 95% confidence interval for this popular vote percentage reported?"

I'm looking at Statistics for dummies...it tells me that to find the margin of error, take the sample size n and find 1 divided by the square root of n. So...1 divided by 22.36 = 0.04--4% margin of error. From there, I want to find the mean, but how do I do this? I have 5 or 6 books I'm looking at, but can't see an exact pattern in any of them.

Answer 1

Try this: http://en.wikipedia.org/wiki/Confidence_interval