Question

Finding Percentile... 1,2,3,4,5,8,9,15
So im doing a sample problem i made up and i end up with 2.7 after using the percentile formula. However it isnt listed in the data, so i decided to average the 2nd and 3rd values and got 2.5 which also is not in the data.. So im wondering what am i doing wrong?

Answer 1

Oh and im looking for the 30%

Answer 2

how many numbers are there?

Answer 3

8

Answer 4

then a percentile is 8/100; or simply .08

how many of those do we need? 30 right?

Answer 5

.08(30) = 2.4

we like decimals, they make this easier; round up

Answer 6

Oh interesting approach. I was using v= 30/100 (n +1)

Answer 7

ah, a sample ... i considering a population

Answer 8

same concept tho :) it looks like we want the (2.xx position) so the 3rd position would be the 30th percentile

Answer 9

Oh gotchya. Basically everything below the 3rd value falls in the 30th percentile.. Thanks i get it now =)

Answer 10

good luck :)

Answer 11

if we didnt have a decimal, the we would have to average some positions

Answer 12

Like if it was 2.7997979797977979 or whatever?

Answer 13

lol, no

say we had 100 numbers (just to make this simple)

100/100 = 1, 30% is 30

the 30th position needs to be included inside of the 30th percentile, so we would have to interpolate between the 30th and 31st position values

Answer 14

ideally that would be (30th + 31st)/2, but different texts like different exactnesses

Answer 15

I see what your saying but i feel like i would have trouble distinguishing when to average or when to round up.

Answer 16

if we have a decimal expansion ... we round to the next position since there is no say: 2.34675 th position

if our nth percentile number is an interger, we need to average the nth and (n+1)th positions

Answer 17

its like finding the median; the number in the middle is either in the middle, or the average of the 2 on the side of the middle

Answer 18

Ok. So if i got 2 id need to find the average between that and the next value. But if i get a decimal i just round up.

Answer 19

correct

as an exercise lets include a "m"edian in your set, and find the 50% percentile of the 9 values

1,2,3,4,m,5,8,9,15

9/100 * 50 = 4.5 ; our median is "m" where we expect it to be

lets take it out and find the 50% of 8 values :)

1,2,3,4,5,8,9,15
^

8/100 * 50 = 4 , we want to average the 4th and 5th postions

Answer 20

This totally makes sense. 4.5 Is the m

Answer 21

Everything below that falls into the 50th percentile

Answer 22

Thx again

Answer 23

youre welcome :)