# Help finding percentiles.

• Jan 21st 2013, 09:26 AM
anthonye
Help finding percentiles.
Hi;
Lets say I have a list of values 1 2 3 4 5 6 7 8 9 10.
and I want to find the 20percentile I whould do this.

20/100*10=2 position two which just happens to have the value of 2 thats fine.

Now lets say I have this list of values 1 2 3 4 5 6 7 8 9
and I want to find the 20 percentile I whould do this.

20/100*9=1.8 because this now produces a decimal do just round up to position two
which just happens to have the value of 2.

because if I find the average of the first ad secund value I get a decimal again ie 1.5.

Conclution: If theres an even number of values just take that value and if theres an odd
number of values round up and take that value?

Thanks.
• Jan 21st 2013, 06:39 PM
chiro
Re: Help finding percentiles.
Hey anthonye.

You need to get the actual value that corresponds to the particular percentile.

So basically you need to find P(X <= x) = a for some x given a value of a (20th percentile implies a = 0.2).
• Jan 22nd 2013, 12:19 AM
anthonye
Re: Help finding percentiles.
Thanks Chiro;
I know I need to find the value of a particular percentile,What I need to know
is am I using the correct process to find a percentile.
• Jan 22nd 2013, 12:30 AM
chiro
Re: Help finding percentiles.
First you rank the observations in ascending (increasing) order and find the probability distribution (from the frequency table).

Once you have done this then you find the x such that P(X <= x) = a where a is your percentile.

This is the general method for finding percentiles.