A Boxplot, also known as Box and Whiskers Plot, is a good way to visually depict Variation in a dataset (e.g., a Sample or Population). And showing several Boxplots vertically is useful for comparing Variation among several datasets. The boxes depict the range within which 50% of the data falls for each dataset. - The bottom of the box identifies the 25th percentile (25% of the data is below)
- The line in the middle is the Median (50th percentile)
- The top of the box is the 75th percentile
- The line segments (the "whiskers") at the top and bottom extend to the highest and lowest values of the dataset. The whiskers are drawn to extend only as far as 1.5 box lengths. (If there are no data points that far out, the whisker ends at the farthest point.) Points beyond 1.5 box lengths are termed "Outliers". Points beyond 3 box lengths are called "Extremes" or "Extreme Outliers".
In this illustration, a higher score is better. Treatment A has the highest individual score, but it has considerable more Variation in results than Treatments B and C. The Medians for Treatments A, B, and C are fairly close. So, we can see at a glance that Treatment D can be eliminated from consideration. Treatment B has the highest Median and is gives very consistent results (small Variation). So, this plot may be all we need to select B as the best treatment.
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## AuthorAndrew A. (Andy) Jawlik is the author of the book, Statistics from A to Z -- Confusing Concepts Clarified, published by Wiley. ## Archives
December 2018
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