00:01
All right, this question gives us sample size, standard deviation, sample mean, and it wants us to generate various levels of confidence with this.
00:11
So remember that the formula for a ci confidence interval is x bar, our point estimate, plus or minus, our margin of error, which is a special z score called z.
00:30
Z star times sigma over square root of n, which is our standard error.
00:39
Which if you look at this formula, it's just our sample mean plus or minus a z score that corresponds to a certain number of standard deviations times our standard error.
00:54
Because the link with the percentages is that this gives the area under the curve corresponding.
01:05
So, now we can fill on the values that won't change, depending on our confidence interval, which is everything except z star.
01:17
Because all these other values were given.
01:23
So, now all we have to do to compute each.
01:26
Confidence interval is find z star.
01:29
So part a wants us to find the 90 % confidence interval.
01:37
So if you look in the table in your book, you can find that z star is equal to 1 .645 for a 90 % confidence interval.
01:52
Or if you want to find this by hand for the first time to see why it's the case, just do inverse norm with an area of 0 .95.
02:04
And now we can just plug into our formula...