Box-and-Whisker Plots

Easily compare quality performance and quickly identify opportunities to bring processes in spec—and on target.

SPC DEMO

Minimize Production Costs, Quickly Detect Issues, and Optimize Your Product Quality

Don’t miss out! Book a demo of our specialized SPC software and unlock immediate improvements in your processes.

Book a Demo

What is a Box-and-Whisker Plot?

A box-and-whisker plot is a well-known statistical process control (SPC) comparative analysis tool that can help you eliminate process variation. Use box-and-whisker plots to compare product and process performance, even on different lines or in different plants.

Like a histogram, box-and-whisker plots reveal the distribution of data values. Instead of a histogram’s frequency distribution, box-and-whisker plots represent the distribution with percentiles.

Box-and-Whisker Plots Explained

Vertical lines on box-and-whisker plots represent percentiles. The leftmost point on each horizontal line (or “whisker”) represents the minimum value while the rightmost dot represents the maximum value. The line in the center of each box represents the 50th percentile. The box itself spans from the 25th to the 75th percentile. The vertical lines on the whiskers represent the 5th and 95th percentiles.

Use Box-and-Whisker Plots for Rapid Insights

In a screen display, box-and-whisker plots are more compact than histograms, enabling several plots to share the same screen space. This means you can easily compare multiple plots. You can quickly compare central tendency and variability for each data set represented by box-and-whisker plots.

Compare Against Specification Limits

To extract even more valuable information, box-and-whisker plots can be compared against specification limits. This allows the viewer to understand which data sets are generating the most (and least) out-of-specification issues. As a result, box-and-whisker plots are ideal tools for prioritizing quality activities and Six Sigma projects.

Compare Multiple Process Streams

Box-and-whisker plots are particularly useful for viewing the performance of multiple process streams in the same interface. This capability is useful in any industry that needs to compare performance between production lines, product codes, and shifts.

For example, in food packaging organizations, minimum fill weights are regulated. Underfilling can result in fines or sanctions; overfilling can significantly increase costs.

Box-and-whisker plots quickly reveal vitally important information such as:

• Whether products are being filled to minimum required levels
• The presence and amount of overfill
• Which products run best on which production lines
• Which production lines run the same product weights higher (or lower) than others
• Whether different shifts fill consistently
• Average fill volumes
• Variability in fill volumes
• Where out-of-specification issues occur the most

Take Another Look at In-spec Data

With the box-and-whisker chart, you can easily discover best practices and opportunities for improvement. Plus, you can examine in-spec data to find savings you might never have expected.

See the Box-and-Whisker Plot in Action

InfinityQS® SPC software solutions provide fast and easy access to box-and-whisker charts, without a great amount of time-consuming manual data entry or analysis. See how easy it can be to spot trouble—or opportunities—by surfacing this information from within SPC software.

See how InfinityQS surfaces this information and makes SPC easy.

Speak to a Distribution Industry Expert

What to Expect

• Free 20-minute call with a product expert
• Explore which solutions best suit your needs
• No-pressure conversation
• Get a live, personalized demo

---

Additional reference material

Additional sections from legacy whiskers-plot:

• Quality Advisor
• Data Collection Tools
• Data Analysis Tools
• Formulas and Tables
• Glossary
• Additional Resources

Whiskers plot

This chart shows the high and low data values and the averages by part and by appraiser. The vertical line represents the range deviation made by an appraiser on one part. This helps determine measurement consistency by an appraiser, across appraisers, and shows abnormal readings, and part appraiser interaction.

To create a Whisker plot:

1. Plot the high and low data values and the average by part for each operator.
2. Draw a line to connect the high value to the low value.
3. Connect the averages for each part for each operator, as shown below.

The longer the line, the larger the deviation from the true value of each part. The Whisker-Box Plot also lets you compare the results of each operator. If one operator’s results vary greatly, the operator may need more training on the measurement techniques and practices.

Whiskers plot

This chart shows the high and low data values and the averages by part and by appraiser. The vertical line represents the range deviation made by an appraiser on one part. This helps determine measurement consistency by an appraiser, across appraisers, and shows abnormal readings, and part appraiser interaction.

To create a Whisker plot:

1. Plot the high and low data values and the average by part for each operator.
2. Draw a line to connect the high value to the low value.
3. Connect the averages for each part for each operator, as shown below.

The longer the line, the larger the deviation from the true value of each part. The Whisker-Box Plot also lets you compare the results of each operator. If one operator’s results vary greatly, the operator may need more training on the measurement techniques and practices.