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