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 the population of birth weights of all babies born in this hospital follow a Normal distribution, with mean μ.

The standard error of the mean is
A. 6.50.
B. 1.53.
C. 0.36.
D. 0.02.

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 the population of birth weights of all babies born in this hospital follow a Normal distribution, with mean μ.

The standard error of the mean is
A.	6.50.
B.	1.53.
C.	0.36.
D.	0.02.

tkhunny · Answer

Have you considered $\dfrac{6.5\;oz}{\sqrt{18}}$?

anonymous · Answer

I am confused as to when to use the standard error formula ( s/√n) . So if I just take 6.5 and divide by √18 that comes to 1.532. However I have a followup question that asks for the 95% confidence interval. When I use my CI formula I get the answer of 119.6 ± 3.00 ounces using the straight σ from the problem. Is that correct, or do I need to use the standard error result for sigma? Thanks.