Question

An SRS of 18 recent birth records at the local hospital was selected.   In the sample, the average birth weight was 119.6 ounces and the standard deviation was 6.5 ounces. Assume that in the population of all babies born in this hospital, the birth weights follow a Normal distribution, with unknown mean μ and unknown standard deviation σ .

We want to estimate μ based on our sample.

A 95% confidence interval for the population mean birth weight based on these data is           

	A.	113.1 to 126.1.
	B.	116.6 to 122.6.
	C.	116.4 to 122.8.
	D.	118.8 to 120.4.

Refer to question number 2: The standard error of the sample mean is                        

	A.	0.36.
	B.	6.5.
	C.	3.2.
	D.	1.53.

Answer 1

1)

t critical value at 0.05 level at 17 df = 2.110

95% confidence interval for is

- t * S / sqrt(n) < < + t * S / sqrt(n)

119.6 - 2.110 * 6.5 / sqrt(18) < < 119.6 + 2.110 * 6.5 / sqrt(18)

116.4 < < 122.8

The 95% CI is from 116.4 to 122.8

2)

Standard error = Standard deviation / sqrt(n)

= 6.5 / sqrt(18)

= 1.53