00:01
Here we are going to be finding a 99 % confidence interval for some blood pressure readings.
00:06
So when we look at our situation here, it tells us that we have sampled 70 people and that they had a mean of 95.
00:15
And it also tells us that we know that the population standard deviation is 11.
00:20
Since we know the population standard deviation, that means we're going to use a z confidence interval rather than a t confidence interval.
00:26
So the z confidence interval for means formula looks like this.
00:37
We've got our x bar plus or minus our z star times sigma over the square root of n.
00:48
And your z star doesn't depend on anything except for your confidence level.
00:53
So if we're trying to make a 99 % confidence level, we can look that z star level up.
00:59
And our z star for a 99 % confidence level will always be 2 .576.
01:07
So that's my z star.
01:09
My sigma it tells us is 11.
01:13
N is 70.
01:16
Right? so, and then our x bar is 95.
01:26
So we are going to just go ahead and put those numbers into our calculator now.
01:34
I'll do my 2 .576 times 11 divided by the square root of 70.
01:44
Put that into my calculator, and i get 3 .39.
01:55
Looks like we want three decimal places, so let's actually make that 3 .387.
02:02
There we go.
02:04
And we still got our 95 here.
02:06
So to find our confidence interval, we'll just go ahead and add and subtract that from 95.
02:12
So i'll do 95 minus 3 .387 to get my 91 .613.
02:21
Right.
02:22
And then 95 plus 3 .387 and i'll get 98 .387.
02:30
Okay...