A Multi-Rule Shewhart Chart For Quality Control In Clinical Chemistry
Quality control in clinical chemistry is essential to ensure accurate test results and patient safety. The use of statistical process control (SPC) charts is a common practice in clinical laboratories to monitor the quality of their testing systems. Among the SPC charts, the Shewhart chart is the most widely used tool for detecting systematic errors and random errors that can affect the accuracy and precision of laboratory results. However, the traditional Shewhart chart has limitations when it comes to detecting small shifts or trends in the data. This is where the multi-rule Shewhart chart comes into play.
What is a Multi-Rule Shewhart Chart?
The multi-rule Shewhart chart is an extension of the traditional Shewhart chart that uses additional rules to detect small shifts and trends in the data. The multi-rule chart is more sensitive than the traditional chart and can detect changes that may not be visible on a traditional chart. The multi-rule chart uses a set of rules to signal when the data points are outside the expected range. The rules are based on the number and pattern of data points that fall outside the control limits.
How Does a Multi-Rule Shewhart Chart Work?
The multi-rule Shewhart chart works by comparing the test results against the control limits that are based on the mean and standard deviation of the data. The chart has six rules that signal when the data points are outside the expected range. The rules are based on the number and pattern of data points that fall outside the control limits. The rules are:
- One point outside the 3σ limit
- Two out of three consecutive points outside the 2σ limit on the same side of the centerline
- Four out of five consecutive points outside the 1σ limit on the same side of the centerline
- Eight consecutive points on the same side of the centerline
- Fourteen consecutive points alternating up and down
- Fifteen consecutive points within the 1σ limit
When a rule is triggered, it signals that there is a problem with the testing system that needs to be investigated. The multi-rule chart is more sensitive than the traditional chart and can detect small shifts and trends in the data that may not be visible on a traditional chart.
Advantages of a Multi-Rule Shewhart Chart
The multi-rule Shewhart chart has several advantages over the traditional Shewhart chart. Some of these advantages are:
- More sensitive to small shifts and trends in the data
- Can detect problems that may not be visible on a traditional chart
- Reduces the risk of false alarms
- Provides a more comprehensive picture of the testing system
- Helps to identify the source of the problem
- Can be used for different types of data distribution
Disadvantages of a Multi-Rule Shewhart Chart
Despite its advantages, the multi-rule Shewhart chart has some limitations. Some of these limitations are:
- Requires a large amount of data to establish the control limits
- May be too sensitive to small changes in the data
- May produce false alarms if the rules are too strict
- May not be suitable for data that is not normally distributed
- Requires a skilled operator to interpret the results
Conclusion
The multi-rule Shewhart chart is a powerful tool for quality control in clinical chemistry. It is more sensitive than the traditional Shewhart chart and can detect small shifts and trends in the data that may not be visible on a traditional chart. The multi-rule chart is a valuable addition to the quality control toolbox of clinical laboratories and can help to improve the accuracy and precision of laboratory results. However, the use of the multi-rule chart requires a skilled operator who can interpret the results and take appropriate action when a rule is triggered.