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# S1 quartile values help Watch

1. Hi, I know this is very stupid of me to ask but I wasnt taught when to do this, but I need some help...

I know how to inrepolate and and find the values, I just never know when to add 1 onto the total number before I divide by 4 or 2 or whatever. It loses me a lot of marks and I'm wondering if anyone would like to explain.

E.g.

out of 200 values, find the median

so do I do 200x2/4?
or do I do (200+1)x2/4?

I feel like an idiot, but its the only part I lose marks at!
2. Oh, and what about grouped data?
3. (Original post by i need ferrets)
Hi, I know this is very stupid of me to ask but I wasn't taught when to do this, but I need some help...

I know how to interpolate and and find the values, I just never know when to add 1 onto the total number before I divide by 4 or 2 or whatever. It loses me a lot of marks and I'm wondering if anyone would like to explain.

E.g.

out of 200 values, find the median

so do I do 200x2/4?
or do I do (200+1)x2/4?

I feel like an idiot, but its the only part I lose marks at!
This might well vary between 'exam boards'. You'd better check mark schemes or specifications....or just ask your teacher.
4. If you're interpolating, you do not need to use the +1 method. It is normally correct to just use n/2 which will give you some number, and find that term. This is generally more accurate.

I also suggest checking with your teacher though because it might be different somehow for your board.
5. This is what we got taught:

For non-grouped data (e.g stem and leaf)

n/2, if it gives an answer to .5, round it up, if it gives a whole number, round it up to its .5,
e.g
10/2 = 5 => 5.5th value
11/2 = 5.5 => 6th value.

Same rule for 3n/4 (Q3)

and n/4 (Q1)

With grouped data, you do not do this rounding rule, the value you need would simply be n/2, etc.

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Updated: December 30, 2010
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