Key Points

• Tolerance ranges are the extreme limits that remain acceptable for production output.
• They can be established with upper and lower control limits on a control chart.
• Exceeding this range can result in a loss of customer satisfaction.

One of the most important parts of defining a problem in Lean Six Sigma (LSS) is understanding what an acceptable level of variability is within your system and how it compares to how much variability is present in your system today.

The upper and lower statistical limits are essential to understanding how much variation you can expect from your current system compared to how much variation you want out of your current system; understanding and determining these limits are the keys to determining tolerance range (TR).

Overview: What Is the Tolerance Range?

Upper and lower statistical limits are the extremes of a tolerated range, which is the acceptable amount of variation in a particular process. The upper limit is the maximum amount of variation allowed; if the amount of variation in a process exceeds this number, it is considered to be out of control. The lower limit is the minimum amount of variation allowed; if a process’s variations fall below this number, it is also considered to be out of control.

Tolerated ranges are statistical boundaries that represent the range of outcomes for a given process. These ranges are either upper or lower tolerance limits, and they exist because not every process can produce perfect results; some variation is unavoidable, so you need to account for that in your planning.

How Does It Work?

So, we’ve defined what they are, but how do they work? Well, a tolerance range establishes acceptable parameters for your production output. Variation is going to naturally occur in any process, that’s just a part of life. However, something to keep in mind is that anything exceeding those ranges is going to lead to customer trust being lost.

3 Benefits of Tolerance Range

The TR approach has three key benefits: 1) it makes LSS more flexible; 2) it helps you identify areas for improvement, and 3) it allows you to create new products or services.

First, the TR approach makes LSS more flexible by allowing you to identify and work within key parameters, rather than forcing your organization to follow one rigid definition of success. For example, if your organization’s goal is to increase customer satisfaction by 5% across all departments, then the tolerated range approach will let you define success as increasing customer satisfaction by at least 4% in some departments and up to 6% in others.

The second benefit of identifying tolerated ranges is that it helps you identify areas for improvement. This type of data analysis will help you determine which departments are having trouble meeting their targets and which ones are exceeding them. This way, managers can focus their efforts on improving the less-than-ideal performance areas first before moving on to the rest of their departmental goals.

The third benefit of identifying tolerated ranges is that it allows you to create new products or services. If your organization’s goal is to increase customer satisfaction by 5% across all departments, then the TR approach will allow your team members to brainstorm new product features or service offerings that could make a meaningful impact on satisfaction in your department. Understanding what you can do to improve customer retention and loyalty will enable you to create a plan of action that can be implemented over time.

Why Is the Tolerance Range Important to Understand?

The TR for a process is the range of values that the process can operate in within specifications. They are important to understand because:

They give us a way to check if our process is better or worse than it used to be. They allow us to determine if our process is capable of producing goods and services within specifications.

Another important use of the tolerated range is detecting changes in our process. If we notice a change in our TR, we’ll want to look into what might be causing that change and make adjustments as necessary to return to its normal state before continuing to produce goods or services.

An Industry Example of Tolerance Range

One example of the application of tolerances in a professional setting is in the field of construction. When building commercial and residential structures, there are often very specific standards for safety and stability. For example, a residential building may need to be able to stand up to winds of up to 100 mph. This means that the building must be able to withstand forces at that level or higher without collapsing.

To make this happen, architects and engineers will use mathematical models to determine how much force different parts of the structure can support, then design individual components that can withstand slightly more than the maximum force they’ll need to support. If they do this correctly, it’s almost impossible for the building to collapse due to wind pressure alone, even if it’s right at the maximum limit.
This is a TR: the amount by which we allow a component or process variable to vary before it becomes unsafe or unstable.

3 Best Practices When Thinking About Tolerance Range

A tolerated range is a result of the design team and client agreeing on how much a given element can vary from the prototype. The degree of variation that is acceptable will depend on the type of product and service design being developed.
Here are some best practices to consider when determining an appropriate range:

1. Consider the product’s context of use:

In which environments will people be using this product? What kinds of conditions (e.g. temperature, location, frequency) does it need to withstand? What does the user need to achieve?

2. Consider the user experience:

How will users interact with it? How does it need to feel in their hands, or when they look at it? What kind of feedback do users expect to receive from it?

3. Consider the brand experience:

What are some key characteristics that speak to your brand’s personality? For example, if you’re designing a subscription box for dog owners, your brand might be playful and approachable, and your box should reflect that with its color scheme, texture, and materiality.

Other Useful Tools and Concepts

So, what happens if your tolerance limits are outside of scope? Well, you might need to start enacting a corrective action plan to get things back on track. A corrective action plan gives permanent solutions for a given problem while letting you know where employee performance falters.

Additionally, you might need to create a cause-and-effect diagram to get down to the root of an issue. This handy visualization tool looks at all possible causes for a given issue, while also looking at the subsequent effects.

Conclusion

In the end, we see how important it is that the product or service is within a tolerated range. Let’s say a person buys a product and finds out it isn’t within the tolerated range. This would probably make them feel angry and disappointed—they may even want to return the product.

But if the product was within the tolerated range, they would likely be satisfied with their purchase, which would make them much more likely to buy from you again in the future. They may even tell their friends about the great products, leading to the desired cycle of repeat business interspersed with new business.