# How to find a value for the 90% percentile

• Feb 19th 2013, 06:34 PM
bob123
How to find a value for the 90% percentile
Given:
Average value of gas mileage for commuter vans is: 18 mi/gal

Find:
What gas mileage would represent the 90th percentile if the average is 18 mi/gal?

Solution:
So far I have found the corresponding z-score for 90% to be 1.28. However I am having difficulty with the problem because it does not give standard deviation.

All help will be appreciated. Thank you.
• Feb 19th 2013, 06:59 PM
chiro
Re: How to find a value for the 90% percentile
Hey bob123.

What is the variance or standard error for the mileage? (You need that to get the percentile)
• Feb 19th 2013, 07:12 PM
bob123
Re: How to find a value for the 90% percentile
This is why I'm confused. None of that is given, nor does it say to make any kind of assumptions other than gas mileage is assumed to be normally distributed. Here it is straight from the textbook:

Attachment 27149
• Feb 19th 2013, 07:37 PM
chiro
Re: How to find a value for the 90% percentile
You can't figure this out without information for the variance component (population or sample).

You either have to assume it, or measure it.
• Feb 19th 2013, 07:45 PM
bob123
Re: How to find a value for the 90% percentile
I'll find out of there's a value we're supposed to assume.

Thanks for the information.