00:01
So we're looking at the lifetime of a pacemaker, and it says that the standard deviation of the standard deviation of lifetimes, the battery is supposed to be less than 1 .8 months.
00:14
That's what the standard deviation should be.
00:16
They took a sample of 20 batteries and found that the sample standard deviation was equal to 1 .6 months.
00:25
And we want to assume that we have a normal distribution.
00:27
We want to find a 95 % percent.
00:30
Confidence interval for the actual standard deviation and so we need to look up in our table if we have 0 .95 here we have 0 .025 in the up lower and upper tail and this area down here is 0 .975 and we need to look this up with degrees of freedom being 19 so let's look at our table and we'll write those down in green hopefully so for 19 degrees of freedom, that lower number is 8 .907.
01:09
8 .907.
01:12
And with 19 degrees of freedom, having 0 .025 in that upper tail, that corresponds with 32 .852.
01:21
30, let me quick look at it again, 32 .852.
01:27
32 .852.
01:29
And then we can calculate our intervals...