Nayland College Mathematics - 'More than just a school'  
 
 

 

 
 
 

Comparing Box plots

1.10 Multivariate Home | Assessment Criteria | The Statistical Cycle | Form a Question | Collecting Data | Cleaning Data | Stem & Leaf Plots | Dot Plots | Central Tendency | Measures of Spread | Box Plots | Comparing Boxplots | Describing Features | Sampling from a Population | Population Inference | Revision

15



Comparing Box-&-Whisker Plots

Comparing groups using box and whisker plots and drawing conclusions.

"No overlap in spreads" or "75% is below 75%" so there IS a difference between group 'A' & 'B'

"Boxes overlap but not both medians" or "75% below 50%" so there IS LIKELY to be a difference between group 'A' & 'B'

"Boxes overlap with both medians" NO difference can be claimed

Using the 'Overall visible spread' to make a comparison when the boxes overlap: The distance between medians DBM as a percentage of Overall visible spread OVS (excellence)

Demo of OVS (allow macros to use slider bars)

Overall visible spread rules
Class notes, Blank notes

pg281
Ex 21.01 pg284

pg 266 to 272 Ex 17.01 pg 273

 

When comparing two groups a box-and-whisker plot is used
A Sample size of at least 30 is needed to generalize about a population

How can we tell if the groups are different?   (Achieve & Merit)

"No overlap in spreads" or "75% is below 75%" so there IS a difference between group 'A' & 'B'

“B is greater than A”   

"Boxes overlap but not both medians" or "75% below 50%"so there IS LIKELY to be a difference between group 'A' & 'B'

“It is likely that B is greater than A” 

"Boxes overlap with both medians" NO difference can be claimed

We cannot tell if there is a difference

 

 

(Excellence) Using the 'Overall visible spread' to make a comparison when the boxes overlap:

The Distance Between Medians (DBM) as a percentage of Overall Visible Spread (OVS)

For a sample size of 30 if this percentage is over 33% there tends to be a difference between A and B

For a sample size of 100 if this percentage is over 20% there tends to be a difference between A and B

For a sample size of 1000 if this percentage is over 10% there tends to be a difference between A and B

Describe the differences between these groups

(Sample of 30)

 

 

back to top