 Hi, this is Dr. Don. I have a problem out of Chapter 6 on confidence intervals. And in it we're asked to find the critical value of z necessary to form the confidence interval given a level of confidence c. And here we're given a value c equal 0.85. We need to find out what that appropriate critical value of z is. If you remember when we're doing confidence intervals the c represents the area that's clustered around the middle of the mean in this case. And we need to find the critical value of z that is located at these transition points. We've got c in the middle so that means the area over here is one half of one minus c. And down here is also one half of one minus c. Now generally when we're doing confidence intervals for this course, Business 233, we want to find the positive critical value. So let me show you how to do that using Excel. Okay, I have Excel open. I've done a little bit of preliminary work here. You know I like to keep things neat and formatted so I can find things. I've labeled in column A the confidence level c one minus c, one minus c divided by two and the critical value of z. Now here's our first confidence level 0.85. To get the one minus c, we just click in that cell equal one minus. I'm going to go up here and click on that cell. And that gives me my one minus c value of 0.15. To get that divided by two half of it, I just click again in the cell equal, click on that cell divided by two and hit enter. And that gives me the value of the area in that far right corner of the tail. To get the critical value of z, we're going to use the norm s inverse function. Just start typing equal in ORM for normal distribution. And we want the s inverse because it returns the inverse of the normal distribution, standard normal distribution, if we're given a probability or an area. So I'm going to double click that to insert that formula function and it asks for the probability. And I'm going to click up there in that cell and hit enter. And whoops, we get minus 1.444, which is the lower n value of the critical value. Remember in our standard normal tables and also in Excel, the value that comes out of the standard functions is always cumulative area from the left tail. Now we want the right tail critical value. So I'm going to go back in here and just click insert ABS for absolute value. Remember absolute value just returns the positive side, the essence of that. So I'm going to close that with the parentheses, hit enter. And that gives us our critical value of 1.44, which is the answer they want up there. Now let's reload and get a different value. And this time it's 0.95. So all I have to do since I've got this set up is just enter 0.95 for my C value. And I've got a new critical value of 1.96, which is the value they want there. And that's the beauty of setting these things up in Excel, labeling them. I'll show you one final thing. Remember we can expose the formulas by using the equal FORM formula text. Double click that to select it. The cell reference I want is that one. Click enter. And that gives me my formula in that cell. I'm going to drag down to get the formulas in the other three cells. So use this tool. Hope this helps.