Finding the Top 10%

Let's say you wanted to find the top 10% of a group of numbers. That is, you wanted to figure out what sample mean is the dividing line between the top 10% of the numbers and the lower 90% of the numbers.

In statistics, the sample's mean is represented by Xbar (X with a bar over it).

The formula is:

z = (Xbar - mean) / (stdev / sq root of n)

so we would say

1.28 = (Xbar - mean) / (stdev / sq root of n)

That means

1.28 * (stdev / sq root of n) = (Xbar - mean)

and finally

(1.28 * (stdev / sq root of n)) + mean = Xbar

Statistics Basics

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