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Knowledge Base » SigmaXL Toolbox

Interpreting Boxplots with SigmaXL

on January 01, 2012

How do I interpret a Boxplot?

Interpreting Boxplots with SigmaXL

SigmaXL Boxplots are a quick check for outliers in your data.

SigmaXL Boxplots now include an optional legend.

Check Show Legend to display the legend with your results:

The height of the box is called the Inter-Quartile Range (IQR) and is a robust measure of spread or sample variability.

Data points highlighted with a circle are potential outliers (< Q1 – 1.5 * IQR or > Q3 + 1.5 * IQR.)

Extreme outliers are highlighted with a solid dot (< Q1 - 3 * IQR or > Q3 + 3 * IQR).

If the data is normal, the probability of a potential outlier (due to random variation) is less than 1 in a 100 and the probability of an extreme outlier is less than 1 in 400,000.

Boxplots should be used in conjunction with control charts: Boxplots are a great quick check for outliers, but control charts tell you when the outliers occur.

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