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# just a (simple?!) question?! watch

1. hey all ~

say i have 90 in a sample and i needed to find the 10th percentile.

np=90*0.1
np=9.

does this mean i have to take the 9th number in the sample? (after sorting it out, smallest to largest)

thanks guys - please help me cos im so confused: will tell more once someone has answered...(i have to ask this question first, just to make sure im doing the right thing lol)
2. Yup, if you want the 10th percentile of 90, you take the 9th number in the sample once ordered.
3. and i am guessing that the same works for the 50th percent (median)....

well. if the sample size was 90 again and using the same method i needed to find the median, i would do np=90*0.5, which is 45.
Therefore i would read off the 45th number in the sample, once its been sorted ascendingly?
4. ^which isn't right, because the middle number would actually be the 45.5th number? which means that i should be taking the average of the 45th and the 46th number.

which way should i use?!
5. 45th number, I believe.
6. yeah thats what i initially thought too....but thats the wrong answer

am working it out by hand and by a computer stats package and the correct answer is the average between the 45th and 46th number to be the median.

thats why im so confused :'(
7. Well in my S1 notes, I have written down that the median of the sample is 50th percentile, which would be the 45th number.

Maybe someone else could clarify.
8. thanks for your help so far.

thats what i originally thought too...but doing it the basic way again (sorting out the numbers the crossing off each one on either end until they meet in the middle), there was no middle number cos it was an even sample: i had to average the 45th and 46th numbers.
9. So do the answers given equal to the average of the n/2 and (n + 1)/2 numbers?
10. for the median it was the average of the (n/2 + (n+1)/2), yes
11. I think we can assume that the median percentile is the (n + 1)/2 th term then.
12. yep, but then its stupid cos on the notes we were given it doesn't say that im so going to take this up with the person who gave me the notes!!!!!!

thanks matt2k, for reassuring me that i wasn't going insane and that i CAN work out a silly S1 level question!!!!!!!
13. Just out of interest, which notes are they? Where did you get them from?

S1 is boring.
14. For the k'th percentile, start at 1 and go k% of the way towards 90.

You end up at the 1 + 0.01k*89 = 1 + 0.89k th entry.

Examples:
k = 50 (median): 45.5th entry [ = (1/2)(45th entry + 46th entry) ]
k = 10 (10th percentile): 9.9th entry [ = (1/10)(9th entry + 9*10th entry) ]
15. (Original post by Jonny W)
For the k'th percentile, start at 1 and go k% of the way towards 90.

You end up at the 1 + 0.01k*89 = 1 + 0.89k th entry.

Examples:
k = 50 (median): 45.5th entry [ = (1/2)(45th entry + 46th entry) ]
k = 10 (10th percentile): 9.9th entry [ = (1/10)(9th entry + 9*10th entry) ]
thats the way to do it i believe

statistics is so convoluted....
16. you lot are LEGENDS! reps for all of you! (one at a time though!)

so for the 90th percentile, using symmetry, it should be the 80.1th entry?

am wondering how to work it out though? still can't get my head quite around it.

k = 90
(9/10)(81st entry + ...
i need to make 801 but i don't know how to...
17. (Original post by posh_git)
you lot are LEGENDS! reps for all of you! (one at a time though!)

so for the 90th percentile, using symmetry, it should be the 80.1th entry?

am wondering how to work it out though? still can't get my head quite around it.

k = 90
(9/10)(81st entry + ...
i need to make 801 but i don't know how to...
90th percentile
= 81.1th entry . . . . . same distance from 90 as 9.9 is from 1
= (1/10)[9(81th entry) + (82th entry)]
18. oh oh oh! i totally get it now!!!!!!!!!

thanks a lot people!!!!!!

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