Boxplot

Overview | How to | Example

Overview

The boxplot (also called the box-and-whisker plot) represents the distribution of data as a box and whiskers. It is useful for comparing the distribution of various groups of data.

Outlier - Unusually small or large observation Upper control - Upper limit which is = Q3 + 1.5 (Q3 - Q1) Q3 - Third quartile, where 75% of the data are less than or equals to this value Q2 - Median, where 50% of the data are less than or equals to this value Q1 - First quartile, where 25% of the data are less than or equals to this value Lower control - Lower limit which is = Q1 - 1.5 (Q3 - Q1)

How to

At the Excel Menu (For Excel 2007, go to Add-ins first)

1. Choose ProcessMA > Graphs > Boxplot

2. For data in single stacked column

   • In Variable, select the column containing the data (Numeric)

   • In Subgroup, select the column containing subgroup indicators (Optional)

3. For data in different columns

   • In Variables, select the columns containing the response data (Numeric, >= 2 variables)

4. Check Show Outliers, if you want to plot outliers, which are represented as *

5. Check Show Median Label, if you want to show median values

6. Check Show Mean Label, if you want plot and show mean values

7. Click OK

Example

A company makes steel bolts using four different machines. You want to assess if the dimensions of the steel bolts made by different machines are the same. You want to use Boxplot to compare the distributions.

1. Open data worksheet by choosing ProcessMA > Tools > Data

2. Choose ProcessMA > Graphs > Boxplot

3. In Variable, select B - Dimension

4. In Subgroup, select C - Machine

5. Check Show Outliers

6. Check Show Median Label

7. Check Show Mean Label

8. Click OK

Results & Interpretation

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