00:01
For this problem, we know that we want a maximum margin of error of 0 .14 kilowatt hours.
00:08
So we want our margin of error to be less than or equal to 0 .14.
00:11
We know that the standard deviation is equal to 1 .9 kilowatt hours.
00:18
And we have an 85 level of confidence level, or 85 % confidence level or our cl, 0 .85.
00:30
Whereas how large of a sample is required to estimate the mean usage of electricity? so our minimum or sample size is going to need to be greater than or equal to.
00:40
The formula that we use here is z for a one -tail proportion of one minus our level of confidence divided by two.
00:49
We then multiply by our population standard deviation and divide by our desired maximum for the margin of error.
01:02
And then we square that.
01:04
That z value that we're using, our confidence coefficient we can call it, for one minus our level of of confidence over 2 would be greater than or right -tail probability of 0 .15 over 2...