Question 239:

Asked on June 16, 2008

Tags: Z-Score

1

Answer:

No answer provided yet.

You need to start with the z-score for the 75th percentile which you can find by using the percentile to z-score calculator, enter .75 and select 2-sided which provides a z of 1.15. Setup an equation and solve for your score:

mean - some score
------------------ = 1.15
    s/SQRT(20)

Fill in what you know and solve for the unknown score. 

The z-score forumla above is for finding the probability assuming you know the population standard deviation and population mean (we rarely know this in real life, but they like to ask you this in statistics books!). The value in the denominator s/SQRT(n) is called the standard error of the mean and is used when you want the probability above or below a certain mean. If you wanted the probability of a single score above 1.15 you would modify the formula to be (mean - score ) / sd.