# Thread: How can you calculate percentiles from a cdf?

1. ## How can you calculate percentiles from a cdf?

I'm having some trouble understanding how to calculate the percentile from a cdf, i realize that a percentile ( ie. percentile .95) is the value in which 95% of all the probabilities contained within the function fall below - but how do you calculate that value from a function?

the fuction i was given is : ky(1-y) where 0>y>1 , btw this is my first post so i didnt know how to do a greater than or equal sign.. but that's what it's supposed to be! thanks

And if Mr.Fantastic is reading this, you've already saved my life a couple times. So thank you

2. Originally Posted by Jobocop
I'm having some trouble understanding how to calculate the percentile from a cdf, i realize that a percentile ( ie. percentile .95) is the value in which 95% of all the probabilities contained within the function fall below - but how do you calculate that value from a function?

the fuction i was given is : ky(1-y) where 0>y>1 , btw this is my first post so i didnt know how to do a greater than or equal sign.. but that's what it's supposed to be! thanks

And if Mr.Fantastic is reading this, you've already saved my life a couple times. So thank you
I save many lives. Every day.

Is $\displaystyle f(y) = ky(1-y)$ where $\displaystyle 0<y<1$ the pdf?

If so, then first you must get the value of k by solving $\displaystyle \int_0^1 k y(1 - y) \, dy = 1$.

Then the 95th percentile is the value of $\displaystyle a$ such that $\displaystyle \int_0^a k y(1 - y) \, dy = 0.95$.

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