Key Points

• The USL is the highest value a process can hit while still meeting customer needs.
• Specification limits aren’t control limits, but rather something specified before production starts.
• You can see improvements in a project’s yield by paying attention to the USL set.

What are upper specification limits?

During the most recent review of the process control charts, someone noticed that the process fell out of control. Thankfully, it was noted that not a single piece of equipment failed specification checks.

One of the operators asks you, “Why do we have two measurements to monitor, control limits, and specification limits?” Randomly pointing at one of the values on the chart, she asks, “What even is an upper specification limit?”

Overview: What is the USL?

The upper specification limit, or USL, represents the highest limit a measurement or reading can reach and still be acceptable to the customer.

To understand why we need specification limits, it helps to understand control limits. A control limit represents the expectations of a process for a sample to be outside a given percentage range expectation. If we use a 95% range expectation, or +/- two standard deviations, it can help demonstrate the expectation of reading being within a range with 95% certainty.

Specification limits are different from control limits, as they are not calculated using means and standard deviations but are instead values determined by the designers (via the customer) to be within acceptable quality boundaries.

They are what is acceptable to the customer. If the specifications, either upper or lower, are inside of the control limits boundaries, then the process is considered not capable, as the process could be operating within acceptable variation parameters and still deliver a product that is not within specification.

Let’s look at the example below. The X-bar R chart shows the data over some time, the mean, specification limits, and control limits (set at 95%, or +/- 2-sigma). It shows when the process went out of control — during readings 12-15.

When the process was out of control, it ran the risk of being out of specification, though, in this example, it did not pass the upper or lower specification limit ranges, meaning the process was within acceptable parameters for the customer.

Benefits to the USL

Seeing Where It Compares with the Upper Control Limit

If the upper control limit is higher than the upper specification limit, then the process can be considered capable, assuming that the inverse is said regarding the lower control and upper specification limits. If not, the process is considered not capable of reliably providing a product that will meet customer requirements.

Easy Spotlight for Process Improvement Opportunities

Readings that pass or come uncomfortably close to the upper specification limits are easy-to-spot red flags for process concerns. Monitor the process, and investigate whenever a reading is higher than the upper specification limit. There may be special circumstances that cause the upper specification limit to be breached. As such, there may be common causes in play with an out-of-control process.

Yield Improvements Through Comparing LSL with the Mean and USL

Take some time and compare where the upper and lower specification limits compare with the mean. If you see a shift where one specification limit is much closer to the mean, and one is much farther away — you may have an opportunity to adjust the process that helps raise or drop the mean to a value that is closer to the halfway point between the upper and lower specification limits.

Why It Matters

So, why does having an upper specification limit matter? When you’re producing anything, it helps to have a handle on what exactly you’re making. Specifying an upper limit to what is still acceptable to a customer makes sure those needs are being met.

Best Practices for Upper Specification Limits

• Verify with the customer that the correct specification level is set. Sometimes it can be expensive and time-consuming to meet the lower specification level. You might find out that the level was set arbitrarily. You also might find out that the customer does not mind having an even smaller LSL.
• Automate the data collection process as much as possible when collecting data to review. This type of data can require a lot of manpower to collect. Often there is a technological solution that collects the data for far less time and money.
• Be sure to not only regularly measure processes for specification limits data, but to review that data regularly. Many companies collect this type of information but wait too long between data reviews. It results in time lost and money spent. 

Other Useful Tools and Concepts

USLs aren’t the only thing you need to pay attention to during production. Understanding how to fulfill the voice of the business is paramount for guaranteeing your organization’s success. As such, our comprehensive overview of the subject is recommended reading.

Additionally, everyone needs a backup plan. Things go wrong, that’s just part of life. Implementing an Out of Control Action Plan or OCAP, is just one way you can make sure the lights stay on when everything else is going haywire.

Conclusion

The upper specification limit, or USL, represents the highest limit that a measurement or reading can reach. That is while still being acceptable to the customer. 

It’s important to compare with the higher control limit to determine if the system is capable of meeting customer expectations over time. Reviewing this regularly will help maintain quality levels, identify future process improvements, and confirm that customer requirements are met.