Difference Between Variable Control Chart And Attribute Control Chart
Control charts are used to monitor processes and ensure they are operating within acceptable limits. They are a valuable tool for quality control and can help identify any issues that may arise during the manufacturing process. Two common types of control charts are variable control charts and attribute control charts. While both types of charts are used to monitor processes, they are different in several ways.
What Is A Variable Control Chart?
A variable control chart is a statistical tool used to monitor and control the variation in a process. It is used when the process output is measured on a continuous scale, such as weight or length. The chart displays the process mean and the upper and lower control limits, which are calculated based on the process data. The chart also shows any points that fall outside the control limits, which may indicate a problem with the process.
Variable control charts are useful for detecting changes in the process over time. They can also help identify the sources of variation in the process, which can be used to improve the overall process performance.
What Is An Attribute Control Chart?
An attribute control chart is a statistical tool used to monitor and control the quality of a process that produces discrete or categorical data. It is used when the process output is not measured on a continuous scale, such as the number of defects in a product or the number of customer complaints. The chart displays the proportion of nonconforming items or the number of nonconformities per unit of output.
Attribute control charts are useful for monitoring the quality of a process over time. They can help identify any changes in the process that may affect the quality of the output. They can also help identify the sources of variation in the process, which can be used to improve the overall process performance.
Key Differences Between Variable Control Chart And Attribute Control Chart
While both types of control charts are used to monitor processes, they differ in several ways. The key differences between variable control charts and attribute control charts are:
1. Data Type
The main difference between the two types of control charts is the data type they use. Variable control charts are used when the process output is measured on a continuous scale, while attribute control charts are used when the process output is not measured on a continuous scale.
2. Control Limits
The control limits on a variable control chart are based on the process data, while the control limits on an attribute control chart are based on the binomial distribution. The control limits on a variable control chart are calculated using the mean and standard deviation of the process data, while the control limits on an attribute control chart are calculated using the proportion of nonconforming items or the number of nonconformities per unit of output.
3. Ability To Detect Changes
Variable control charts are more sensitive to changes in the process than attribute control charts. This is because variable control charts use continuous data, which allows for more precise monitoring of the process. Attribute control charts, on the other hand, use discrete or categorical data, which can make it more difficult to detect small changes in the process.
4. Types Of Process Variability
Variable control charts are used to monitor both common cause and special cause variability, while attribute control charts are primarily used to monitor common cause variability. Common cause variability occurs naturally in the process and is expected, while special cause variability is caused by external factors and is not expected. Variable control charts are better suited for detecting special cause variability, while attribute control charts are better suited for detecting common cause variability.
Conclusion
Variable control charts and attribute control charts are both valuable tools for monitoring and controlling processes. While both types of charts are used to monitor processes, they differ in several ways, including the data type they use, the control limits they use, their ability to detect changes, and the types of process variability they monitor. Understanding these differences is important for selecting the appropriate control chart for a given process.
