 Hi, this is Dr. Don. I have a problem out of chapter 6 on confidence intervals. I want to show you in this video just how to find critical values for a single sample confidence interval for a mean. I'm not going to go deeply into the theory. I just want to show you how to use the stat crunch technology. You remember, if we're given a problem like this where we're given the confidence level C and we need to find the critical value of Z and we look over here at our standard normal distribution and remember that the C is the area clustered around the mean and so the critical values, there's a negative critical value of Z and a positive critical value of Z because we always look at two-tail confidence intervals in this course and so we have to take half of the area that's not in the middle and put it in each tail, one half of one minus C, one half of one minus C and we're going to use that when we do stat crunch. So we're going to go to question help here and then open stat crunch. Okay, I have stat crunch open and as we do on most of our problems, we will go to the stat button and we want the calculators this time and we're going to go down to the normal calculator because that's how we get Z score, Z statistics. It opens up with the standard normal which is equivalent to the standard normal tables. We have the mean of zero and the standard deviation of one. Now, when we're working with confidence intervals around a single sample mean, we want to get the positive Z for the critical value. Usually that's what they want you to report. So let's click on the drop down and get the right tail which you can see in red there. Now that represents 0.5. Now our area is different. It's one minus C divided by two. Now you can use your calculator, hand calculator to get that or you can just type in a little formula, open parentheses one minus and it's 0.93, 0.93, close the parentheses, divide by two and click compute and you can see we've got our positive critical value 1.812 and that's this little red area there and if we look over here in our problem, you can see that's the value they wanted. 1.81, the positive critical value. Let's reload that and we get a different value of C and that's the reason I wanted to show you this. Many of the tables that you would use, the standard normal tables, would not have an easy way to get something for a confidence level of 0.85 but we can do it in stat crunch again. Pretty simply we just go in here and use our little formula again, open one minus the confidence level 0.85, close the parentheses, divide by two. I've got the greater than symbol pointing to the right tail and click compute and we get 1.39 which rounds to 1.44 which are answers they want. So again you don't have to use this little formula. You can use your little handheld calculator or your phone to find the alpha over 2. I just like to do it that way. So this is a quick way of finding a critical value of Z using stat crunch. Hope this helps.