Quartiles in box plots

Hello, for my coursework I have decided to draw box plots but I am unsure how to work out the values for the quartiles. I have 25 pieces of data in each box plot so for my median value I would do 25+1 = 26/2 =13. However for my lower and upper quartile if I do 26/ 4 * 1 and 26/4 *3 I get 6.5 and 19.5. Would I round these figures up or would I find the middle of the 6th and 7th value and the middle of the 19th and 20th value and just use those?

Reply 1

BabyMaths

Mr M found this for us recently.

Probably not the nice clear answer you hoped for.

Rather than asking what the best method is, perhaps a better question is: "Who is marking your work and how do they expect you to do it?"

Reply 2

SaraB_

OP

Wow ok :/ Well this is for my AQA GCSE statistics coursework and my teacher is marking it. When we did quartiles in class, we always got numbers such as 15 where we were taught to do 15+1 = 16/4 which was a nice easy number that you could find quartiles from. Therefore, I don't really know what to do...

Reply 3

BabyMaths

Original post by SaraB_

Therefore, I don't really know what to do...

Back at school tomorrow? Ask your teacher. :smile:

Reply 4

SaraB_

OP

Original post by BabyMaths

Back at school tomorrow? Ask your teacher. :smile:

Actually the coursework is due in the first day back which is next Monday so unfortunately I won't be getting any help from the teacher. :confused:

Anyway thank you for the help :smile:

Also I was wondering if you could help me on another matter which is to do with histograms and cumulative frequency diagrams. My hypotheses for the coursework are ' the larger the population of an area, the higher the unemployment rate' and ' an area which has a lower NVQ4 rate has more people who are unemployed'. I want to try and include those graphs as they are higher level but I'm not sure how I could do that to compare the variables. I would be very grateful for any suggestions. :smile:

(edited 11 years ago)

Reply 5

BabyMaths

Original post by SaraB_

Actually the coursework is due in the first day back which is next Monday so unfortunately I won't be getting any help from the teacher. :confused:

Anyway thank you for the help :smile:

Also I was wondering if you could help me on another matter which is to do with histograms and cumulative frequency diagrams. My hypotheses for the coursework are ' the larger the population of an area, the higher the unemployment rate' and ' an area which has a lower NVQ4 rate has more people who are unemployed'. I want to try and include those graphs as they are higher level but I'm not sure how I could do that to compare the variables. I would be very grateful for any suggestions. :smile:

OK. In that case, for the quartile problem, I'd suggest you use the mean of the 6th and 7th for the lower quartile and the mean of the 19th and 20th for the upper quartile. That is, find the median of those items below the median etc. This is one of the accepted methods and it's easy to explain.

For your other question, I'll have a think but I can't promise to come up with anything. I dislike GCSE statistics, especially the controlled assessment.

Reply 6

SaraB_

OP

Original post by BabyMaths

OK. In that case, for the quartile problem, I'd suggest you use the mean of the 6th and 7th for the lower quartile and the mean of the 19th and 20th for the upper quartile. That is, find the median of those items below the median etc. This is one of the accepted methods and it's easy to explain.

For your other question, I'll have a think but I can't promise to come up with anything. I dislike GCSE statistics, especially the controlled assessment.

Ok I will do that for the quartiles then, thank you :smile:

I don't like this controlled assessment either and I'd prefer it if the whole GCSE was just the exam as I am finding this quite difficult so thank you again for your help.

Quartiles in box plots

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