Confidence Interval problem.
I'm having trouble figuring out this question.
Suppose that we are interested in buying ball bearings from a certain manufacturer. We require very precisely manufactured ball bearings, and so we decide to investigate this manufacturer's output. In a random sample of 400 of the ball bearings, 48 of them contain manufacturing defects that make them useless to us.
What is a 95% confidence interval for the population proportion of defective balls?
I'm using the equation: .12+ 1.96 * (stnd dev./ square root of n)
I estimated 2.4 as the standard deviation in the question before, but I think that might be wrong.
Also What is the 99% margin of error? I have no clue how to figure that out.
Some guidance on how to at least start this question/equation would be great.