Box and Whisker plots (sometimes just called box plot) provide information about the variability of data and the extreme data values. It is used to compare distributions and get an Instant picture of variation and centring in a dataset, and will show possible outliers.
The plot is based on percentiles of your data, and common reported percentile values are.
- 25th percentile, also called the first quartile (Q1)
- 50th percentile, also called the median
- 75th percentile, also called the third quartile (Q3)
Using Box and Whisker Plot
Box and whisker plots visually show variation in data, which make them very easy to read. They can be very effectively used when comparing data that are related to each other in a way.
For example if you produce an axle that needs to have a bearing mounted on it, but have noticed that sometimes the bearing is loose on the axle. Then a Box and Whisker Plot will tell you, if you have some axles at the lower tolerance that is smaller than bearings at the upper tolerance.
Below is an example of how moisture measurement from 9 labs can easily be compared.
Creating a Box and Whisker Plot
When drawing a box and whisker plot, draw a box which represents the middle 50% of the data. Each end of the box should be at the quartiles and , and a line drawn across the box at the statistical mean.
Following the whiskers are draw from each end of the box to the furthest data point which is not an outlier like seen below.
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