Question

In a study of 225 adults, the mean heart rate was 72 beats per minute. Assume the population of heart rates is known to be approximately normal with a standard deviation of 10 beats per minute. What is the 90% confidence interval for the mean beats per minute?

Answer 1

90% CI z* = 1.64
x bar = 72
n = 225
s = 10
x + - z* (s/sqrt n)
72 + - 1.64(10/sqrt225)
72 + - 1.64 (10/15)
72 + - 1.64(2/3)
72 + 1.64(2/3) = 73.09
72 - 1.64(2/3) = 70.91