# Help Calculating the Z value **URL:** \ **Category:** Factual Questions\ **Created:** [October 25, 2004, 2:56am UTC](https://boards.straightdope.com/t/help-calculating-the-z-value/270984 "2004-10-25T02:56:38Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![saluki\_fan](https://avatars.discourse-cdn.com/v4/letter/s/b2d939/32.png) [@saluki\_fan](https://boards.straightdope.com/u/saluki_fan)\ **Post date:** [October 25, 2004, 2:56am UTC](https://boards.straightdope.com/t/help-calculating-the-z-value/270984/1 "2004-10-25T02:56:38Z")
OK, so I am preparing for a statistics quiz tomorrow and I ran across a problem I should know how to work, but now I am stuck on. “Corssett Truck Rental has a large fleet of rental trucks. At times many of the trucks need substantial repairs. Mr. Crossett has requested a study of the repair costs. A random sample of 64 trucks is selected. The mean annual repair cost is 1200, with a standard deviation of 280. Estimated the mean annual repair cost for all rental trucks. Develop the 95 percent confidence interval for the population mean.” Now I don’t want just the answer to this problem and no, it’s not homework. I realize the first thing I have to do for this problem is first calculate the z value. I thought that I was to take the 1200-64 and take that answer and divide by 280, but I must be wrong, since it says that z value is 1.96 and that is the not the answer the above method yields. With this being said, am I using the right numbers or do I need to calculate some other ones first? Thanks. ---
**Author:** ![ccwaterback](https://avatars.discourse-cdn.com/v4/letter/c/df705f/32.png) [@ccwaterback](https://boards.straightdope.com/u/ccwaterback)\ **Post date:** [October 25, 2004, 3:05am UTC](https://boards.straightdope.com/t/help-calculating-the-z-value/270984/2 "2004-10-25T03:05:21Z")
The answer can be found here: [http://www.netnam.vn/unescocourse/statistics/stat\_frm.htm](http://www.netnam.vn/unescocourse/statistics/stat_frm.htm) ---
**Author:** ![Avumede](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/avumede/32/7235_2.png) [@Avumede](https://boards.straightdope.com/u/Avumede)\ **Post date:** [October 25, 2004, 3:15am UTC](https://boards.straightdope.com/t/help-calculating-the-z-value/270984/3 "2004-10-25T03:15:51Z")
To answer your question directly, the z-value is a value in standard deviations (where the mean is 0 and the standard deviation is 1). It just so happens that the 95% confidence interval corresponds to a z-value of 1.95. You can find this by looking at certain charts. If you have a stats book, this would be in a chart at the back of the book. ---
**Author:** ![saluki\_fan](https://avatars.discourse-cdn.com/v4/letter/s/b2d939/32.png) [@saluki\_fan](https://boards.straightdope.com/u/saluki_fan)\ **Post date:** [October 25, 2004, 3:24am UTC](https://boards.straightdope.com/t/help-calculating-the-z-value/270984/4 "2004-10-25T03:24:54Z")
> [@Avumede](#): > > To answer your question directly, the z-value is a value in standard deviations (where the mean is 0 and the standard deviation is 1). It just so happens that the 95% confidence interval corresponds to a z-value of 1.95. You can find this by looking at certain charts. If you have a stats book, this would be in a chart at the back of the book. So wouldn’t that make it 1.95 instead of 1.96? ---
**Author:** ![saluki\_fan](https://avatars.discourse-cdn.com/v4/letter/s/b2d939/32.png) [@saluki\_fan](https://boards.straightdope.com/u/saluki_fan)\ **Post date:** [October 25, 2004, 3:28am UTC](https://boards.straightdope.com/t/help-calculating-the-z-value/270984/5 "2004-10-25T03:28:26Z")
ok, I think I figured it out. What I should do is take 95% of .5 because that’s half of the range. Take that answer and find it in the chart labeled area under the normal curve and that will give me my answer.