Thread: Confidence interval

Find a 95% confidence interval for the percentage of cars on a certain highway that have poorly adjusted brakes, using a random sample of 500 cars stopped at a roadblock on that highway, 87 of which had poorly adjusted brakes.

I think I need to find k, with



n = 500, .

Not sure how to find c or s.

Try this

Originally Posted by pickslides

Try this

Where did that come from? When I calculated it I got .5063 and -0.1583.


This is the answer, but I don't know how c or was calculated.

For c, I tried and looking up the value for .975 on the t distribution chart with 50 - 1 = 49 degrees of freedom. But 49 was not on the chart.

For , I'm not understanding why it is .