6.4 Percentiles of The Standard Normal Distribution and Percentiles of The Random Variable X
Suppose a random variable 
is 
. To determine the pth percentile of 
, we first need to find the pth percentile of 
To this end, the value of
at the pth percentile, denoted 
has 100 p% of the area under 
below it, that is, 
satisfies the equation
(6.3) 
For instance, to find the 80th percentile of the 
distribution, let us work with the equation
(Fig. 6.27a). Then, since 0.80 = 0.50 + 0.30, let us find 0.30 ...
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