Overview of the SPC Tool

A control chart indicates when your process is out of control and helps you identify the presence of special-cause variation. When special-cause variation is present, your process is not stable and corrective action is necessary. Use control charts to do the following:

• Demonstrate whether your process is stable and consistent over time. A stable process is one that includes only common-cause variation and does not have any out-of-control points.
• Verify that your process is stable before you perform a capability analysis. A capability analysis is only valid when performed on a stable process.
• Assess the effectiveness of a process change. With control charts, it is easy to compare shifts in the process mean and changes in the process variation.
• Communicate the performance of your process during a specific period of time.

You can use a capability analysis to determine whether a process is capable of producing output that meets customer requirements when the process is in statistical control. You can use a capability analysis to answer questions such as:

• Is the variability of a process low enough to consistently provide parts that fall within the specification limits?
• Is the proportion of defectives consistently less than 5% during a month?
• Is a temperature curing process capable across multiple batches of the product?
• Does a process need to be shifted to operate within the specification limits?