Which statement correctly describes the relationship between process stability and process capability?

Which statement correctly describes the relationship between process stability and process capability?

• A. A customer cares more about stability than capability
• B. If a process is stable, it is also capable
• C. None of the above
• D. Process stability does not take into consideration the targets whereas capability does

Answer: (D)

The main idea is the difference between how predictable a process is and how well it can meet its specification limits. Process stability refers to being in statistical control—the process variation is consistent and free of special causes, so the output is predictable over time. It does not involve how close the process average is to the target or how tight the spec limits are. Process capability, on the other hand, measures how well the process could produce within the specified targets given its variation, and it uses the target and specification limits to assess that ability (through metrics like Cp and Cpk). So a process can be stable but not capable if its variation is larger than the allowable range or if it is centered far from the target. Conversely, a process cannot be truly deemed capable if it is not stable, because you wouldn’t trust the capability assessment on unstable data. That’s why the statement that stability doesn’t consider targets while capability does best captures the relationship.

The main idea is the difference between how predictable a process is and how well it can meet its specification limits. Process stability refers to being in statistical control—the process variation is consistent and free of special causes, so the output is predictable over time. It does not involve how close the process average is to the target or how tight the spec limits are. Process capability, on the other hand, measures how well the process could produce within the specified targets given its variation, and it uses the target and specification limits to assess that ability (through metrics like Cp and Cpk). So a process can be stable but not capable if its variation is larger than the allowable range or if it is centered far from the target. Conversely, a process cannot be truly deemed capable if it is not stable, because you wouldn’t trust the capability assessment on unstable data. That’s why the statement that stability doesn’t consider targets while capability does best captures the relationship.