In a study of 250 adults, the mean heart rate was 70 beats per minute. Assume the population of heart rates is known to be approximately normal with a standard deviation of 12 beats per minute. What is the 99% confidence interval for the mean beats per minute? (5 points)

68.9 - 76.3
70 - 72
61.2 - 72.8
68 - 72

Question

In a study of 250 adults, the mean heart rate was 70 beats per minute. Assume the population of heart rates is known to be approximately normal with a standard deviation of 12 beats per minute. What is the 99% confidence interval for the mean beats per minute? (5 points)
 		
	68.9 - 76.3
	70 - 72
	61.2 - 72.8
	68 - 72

anonymous · Answer

someone please help!

kropot72 · Answer

A 99% confidence interval for the mean is:
$$\large (\bar{x}-2.576\frac{\sigma}{\sqrt{n}},\ \bar{x}+2.676\frac{\sigma}{\sqrt{n}})$$
Now you just need to plug the given values into the formula.

anonymous · Answer

So $$\sigma$$ would be 12 but which would be x and which would be n? I know its between 250 and 70 but I dont know which one is which? @kropot72 @tkhunny

kropot72 · Answer

n is the sample size. In this case n = 250.

tkhunny · Answer

$\sigma = 12$

kropot72 · Answer

x-bar is 70.

kropot72 · Answer

A correct to my posted formula. the z-critical value is 2.567 in both terms.

anonymous · Answer

So it would be 68-72 right?

kropot72 · Answer

Correct! Good work.

anonymous · Answer

Thank you so much!! I understand it now!

kropot72 · Answer

You're welcome :)

tkhunny · Answer

kropot72 was paying better attention.  Leave ½% in each tail.  :-)