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? 68.9 − 76.3 70 − 72 61.2 − 72.8 68 − 72

The z score for a confidence level of 99% is 2.575.. You need to know this to complete this type of problem.. the other two commonly used confidence levels are 90% and 95% having z scores of 1.645 and 1.96 respectively. You need to memorize this!

To find the confidence interval, it will be defined as the sample mean plus/minus the margin of error.. Now to find the margin of error, divide the standard deviation of 12 by the square root of the number of elements in your sample(√250). Then take that result and multiply it by the Z score I mentioned above for a 99% confidence level.

In this case: the sample mean is 70 and the margin of error is approximately 1.95

So to calculate the confidence interval, do the following:
70 - 1.95 = 68.05 rounded to the nearest whole number is 68
70 + 1.95 = 71.95 rounded to the nearest whole number is 72

Looks like that would be the last choice. hope this helps.. it's been awhile since my work with statistics :-)