A simple random sample of 100 8th graders at a large suburban middle school indicated that 87% of them are involved with some type of after school activity. Find the margin of error associated with a 95% confidence interval that estimates the proportion of them that are involved in an after school activity.

The confidence interval for the mean is given by the following formula:  

And the margin of error is given by:

Step-by-step explanation:

Previous concepts

A confidence interval is "a range of values that’s likely to include a population value with a certain degree of confidence. It is often expressed a % whereby a population means lies between an upper and lower interval".  

The margin of error is the range of values below and above the sample statistic in a confidence interval.  

Normal distribution, is a "probability distribution that is symmetric about the mean, showing that data near the mean are more frequent in occurrence than data far from the mean".  

Solution to the problem

In order to find the critical value we need to take in count that we are finding the interval for a proportion, so on this case we need to use the z distribution. Since our interval is at 95% of confidence, our significance level would be given by and . And the critical value would be given by:

The confidence interval for the mean is given by the following formula:  

And the margin of error is given by: