Histograms in Six Sigma Projects: Understanding Variation at a Glance

In Six Sigma projects, data drives decisions. One of the most powerful yet simple tools to understand your data is the histogram. Whether you're trying to reduce defects, improve cycle time, or enhance customer satisfaction, a histogram can provide instant visual insight into how your process behaves.

This blog will walk you through what a histogram is, how to use it in Six Sigma, and why it’s indispensable during process improvement initiatives.

What is a Histogram?

A histogram is a graphical representation of the distribution of a dataset. It shows how frequently values occur within specified ranges (called “bins”).

Each bar in the histogram represents the number of data points that fall within that range. The height of the bar indicates the frequency.

Why Use Histograms in Six Sigma?

In Six Sigma, the goal is to reduce process variation and improve consistency. Histograms make it easy to:

• Visualize the spread and shape of your data

• Identify skewness, clustering, or outliers

• See whether your data is normally distributed

• Detect potential shifts or patterns in the process

When to Use a Histogram in DMAIC

Histograms are most useful in the Measure and Analyze phases of DMAIC:

Measure Phase

• Assess current process performance

• Understand how much variation exists

• Check if the process is capable of meeting customer specifications

Analyze Phase

• Explore possible causes of variation

• Compare distributions before and after a change

• Validate the impact of different process inputs (X) on the output (Y)

How to Create a Histogram (Step-by-Step)

You can create a histogram using tools like Excel, Minitab, or statistical software. Here’s a basic process:

1. Collect data

• Choose a measurable variable (e.g., delivery time, call handling time, weight).

• Ensure you have a sufficient sample size (30+ data points is a good start).

2. Group the data into bins

• Decide how many intervals (bins) to use—typically 5 to 15.

• Each bin should represent a range of values.

3. Count how many values fall into each bin

4. Plot the data

• X-axis = Bins (ranges of data)

• Y-axis = Frequency (count of data points in each bin)

Example in a Six Sigma Project

Let’s say you’re working on a project to reduce order fulfillment times in an e-commerce business.

You collect data for 50 orders. The histogram shows:

• Most orders are completed in 24–26 hours

• A few outliers take more than 40 hours

• The distribution is slightly right-skewed, indicating that while the average time is reasonable, delays are affecting some customers

Insight:

This visualization helps the team dig deeper into root causes for those delayed orders and target specific improvements.

Interpreting Histograms

Here’s what to look for when analyzing a histogram:

Pattern	What It Suggests
Bell-shaped (Normal)	Stable, predictable process
Right-skewed	Many values are low, but a few are very high
Left-skewed	Many values are high, but a few are very low
Bimodal (2 peaks)	Possible multiple processes or shifts
Flat/Uniform	Random variation, no clear trend
Gaps or Outliers	Data entry errors or rare, special cause events

Tips for Using Histograms in Projects

• Always label your axes and bins clearly.

• Use consistent bin widths to avoid distortion.

• Combine histograms with other tools like Box Plots or Control Charts for a deeper analysis.

• Don’t jump to conclusions—use histograms as a starting point, not the final verdict.

Tools to Create Histograms

• Excel: Easy and accessible using the Data Analysis Toolpak

• Minitab: Provides more advanced statistical visuals

• Google Sheets: Simple histogram chart option

• Python/R: For automated analysis on larger datasets

Final Thoughts

Histograms are a foundational Six Sigma tool—simple to use, yet incredibly insightful. By helping you visualize the natural variation in your process, they guide your team toward better decisions and more focused improvements.

Remember: If you can’t see the variation, you can’t control it. And if you can’t control it, you can’t improve it.