## 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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