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# Comparing Box plots

 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