00:02
All right, in your question, we're looking at smoking during pregnancy and how does it affect or does it, is it associated with premature births? to investigate that, researchers selected a random sample of 116 pregnant women who were smokers.
00:17
The average pregnancy length for that sample was 255 days.
00:22
From a large body of research, it's known that the length of a human pregnancy has a standard deviation of 16 days.
00:28
The researchers assume that smoking does not affect the variability of pregnancy length, and we're trying to find a 95 % confidence interval to estimate the length of pregnancy for women who smoke.
00:41
And they go ahead and give us a critical value for this, a 1 .96, so that saves us some work.
00:49
All we have to do to build our statistic, or i'm sorry, our confidence interval, is to take our statistic, which is going to be your sample mean, then we plus or minus a critical value, which is often called z star, times standard error.
01:04
Standard error is found by taking standard deviation divided by square root of the sample size.
01:11
So for us, this is going to be 255 plus or minus.
01:16
They gave us the critical value, 1 .96, times standard deviation 16 divided by square root of 116.
01:29
Okay.
01:30
And now we're going to round our decimals to three decimal.
01:32
Places.
01:33
I'm going to first do 255 minus this calculation to get the lower limit of our confidence level...
