A U chart is one of four quality control charts that engineers use to track variation in a manufacturing process. The other three typical quality control charts are the 1.) p chart (which plots the proportion of defective items, 2.) c chart (which shows the number of defects), and 3.) np chart (which shows the number of faulty units).
Overview: U Chart
Quality control engineers use the U chart to track the average number of defects per unit in a manufacturing process. It assumes that defects occur in a statistical Poisson distribution. The “center line” of the U chart is the total number of defects in a sample divided by the number of inspected items. The “control limits” are three standard deviations above and below the center line (average number of defects).
The bottom-line purpose of a U chart is to track defects and help engineers understand what causes them. They are an essential tool in the quality control process, and they allow managers to see how changes in a process affect the number of defects in a sample. Putting defect data on a U chart helps quality control managers and production engineers quickly see changes in how many defects occur in a sample.
Four Benefits of Using a U Chart
There are four main benefits of using a U chart to visualize quality control data:
1. They Help Quality Control Managers Understand Inherent Variations
2. Can Help Managers Identify When Something Is Going Wrong
If the average number of defects suddenly spikes, a U Chart will quickly show quality control managers that a process has gone haywire.
3. Can Show Patterns in Defect Data
Identifying patterns in defect data can help engineers track their causes and improve the process.
4. Helps Predict Future Performance
Average defect data will tell engineers how many defects they can expect to occur using the current process.
Why Are U Charts Important to Understand?
They Can Help Generate Ideas About Improving a Process – Continuous improvement is an important part of lean six sigma management. A U chart helps engineers see when and how often defects occur, leading to ideas about how the quality control process can limit them.
Can Track How Changes to a Process Affect Defects – When engineers attempt to improve a process, they need measurement tools to identify how those changes affect a process. Without a U Chart to track these outcomes, quality control managers wouldn’t be able to know when their efforts were improving a process.
U Charts Can Show How Quality Control Can Adjust a Process – Taking the correct action to limit variations is essential to good quality control. Understanding how to interpret a U chart enables managers to take effective corrective action.
Industry Example of a U Chart
Suppose a car manufacturer wanted to track defects in the windshields they produce. The quality control team would first need to record the average number of defects in a group of samples. This number would provide the “center line” for a U chart, and the team would calculate the standard deviation of the sample. Next, quality control would create upper and lower boundaries for the chart at three standard deviations above and below the mean. Finally, the team would plot points for the number of defects found in each lot in the sample.
Three Best Practices When Thinking About U Charts
U Charts only help quality control if engineers understand how to interpret them correctly. The following three “best practices” will help you know how to use U charts.
1. If Values Extend Beyond the Control Limits, the Process Is Out of Control
The process isn’t predictable if you see defect numbers that exceed three standard deviations from the mean. An “out of control process” is one in which engineers can’t reliably predict how many defects will occur in a production run.
2. You Must Understand the Difference Between Defects and Defective Products
Defective products are units that fail quality control and can’t be delivered to customers or used in the manufacturing process. Defects are product variations that still fall within acceptable limits. While defects are undesirable, these units still meet specifications and can be delivered to customers or used in production.
3. Use U Charts When Products Can Have Multiple Defects
Using a U chart assumes that more than one defect can occur in a unit. If quality control is measuring only one kind of defect, an np chart is a better choice.
Frequently Asked Questions (FAQs) About U Charts
The following three questions are often asked about U Charts:
How Do U Charts Differ From P Charts?
U Charts measure the average number of defects in a unit. Conversely, the p chart shows the proportion of defective items compared to non-defective units. Recall that “defective” means the unit fails to meet specifications. Consequently, p charts aim to track waste, while U charts are about quality.
When Should You Use U Charts and C Charts?
Engineers use the U chart when they are concerned about measuring defects in lots with a variable sample size. Quality control teams use the c chart when the sample size is constant.
What Assumptions Do Engineers Make When Using U Charts?
Quality control engineers make three assumptions when using a U chart, 1.) the probability of a defect is the same for each unit, 2.) each unit is independent of each other, and 3.) the testing procedure is the same for each lot. Engineers should not use a U chart if any of these assumptions don’t apply to a process.
U Charts Are An Important Tool In Quality Control
U Charts are one of four charts engineers typically use to track defects in quality control. The U chart is the go-to tool when tracking defects in lots with varying sample sizes and when products can have more than one defect. However, engineers must remember that they can only use the U chart when defects can be measured in whole numbers.