by Control charts are graphical representations of process data over time, which allow for the identification of patterns and trends in the data. Control charts are used in Statistical Quality Control (SQC) to monitor process performance and detect when the process is out of control.
The construction of control charts for variables and attributes depends on the type of data being monitored. Control charts for variables are used when the data is continuous and measured on a numerical scale, while control charts for attributes are used when the data is discrete and measured in terms of the number of occurrences. By using control charts, organizations can monitor process performance and make informed decisions about process control and improvement.
There are two main types of control charts: control charts for variables and control charts for attributes.
Control Charts for Variables:
Control charts for variables are used to monitor the performance of a process when the data is continuous and measured on a numerical scale. These control charts are used to monitor variables such as weight, length, temperature, or time. The most commonly used control charts for variables are the X-bar and R chart and the X-bar and S chart.
- The X-bar and R chart: The X-bar and R chart is used to monitor the mean and range of a process. The X-bar chart plots the sample means of the process over time, while the R chart plots the sample ranges. If the sample means or ranges fall outside of the control limits, it indicates that the process is out of control.
- The X-bar and S chart: The X-bar and S chart is similar to the X-bar and R chart, but it uses the sample standard deviation (S) instead of the sample range to monitor process variation.
Control Charts for Attributes:
Control charts for attributes are used to monitor the performance of a process when the data is discrete and measured in terms of the number of occurrences. These control charts are used to monitor attributes such as the number of defects, errors, or defects per unit. The most commonly used control charts for attributes are the p-chart and the c-chart.
- The p-chart: The p-chart is used to monitor the proportion of defective items in a sample. The chart plots the proportion of defective items over time, and if the proportion falls outside of the control limits, it indicates that the process is out of control.
- The c-chart: The c-chart is used to monitor the number of defects per unit. The chart plots the number of defects over time, and if the number of defects falls outside of the control limits, it indicates that the process is out of control.
