Process capability refers to a process's ability to produce output that consistently meets customer specifications.

Process capability refers to a process's ability to produce output that consistently meets customer specifications.

The key indicators that show capability are:

• Process Capability Index ($C_p$): This measures the potential capability of a process by comparing the process spread (typically $\pm 3\sigma$) to the total specification spread. A $C_p$ value of 1.0 means the process spread equals the specification spread. Values greater than 1.0 indicate the process is potentially capable of meeting specifications. $$C_p = \frac{\text{Upper Specification Limit (USL)} - \text{Lower Specification Limit (LSL)}}{6\sigma}$$
• Process Performance Index ($C_{pk}$): This measures the actual capability of a process, taking into account both the process spread and its centering relative to the specification limits. It is the minimum of the upper and lower capability indices, reflecting how close the process mean is to the nearest specification limit. A $C_{pk}$ value of 1.0 means the process is barely capable of meeting specifications, while values greater than 1.33 are generally considered good. $$C_{pk} = \min\left(\frac{\text{USL} - \mu}{3\sigma}, \frac{\mu - \text{LSL}}{3\sigma}\right)$$ where $\mu$ is the process mean and $\sigma$ is the process standard deviation.
• Defect Rate/Parts Per Million (PPM): This indicates the number of defective items produced per million opportunities. A lower PPM signifies higher process capability, as it means fewer defects are being generated.
• Statistical Control: Before assessing capability, a process must be in statistical control, meaning its variation is stable and predictable over time, typically monitored using control charts. A process not in control cannot be reliably assessed for capability.

The data showing capability would typically include control charts demonstrating stability, and calculated $C_p$ and $C_{pk}$ values that meet or exceed target thresholds (e.g., $C_{pk} \ge 1.33$). A non-capable process would show $C_{pk}$ values below target, high defect rates, or a lack of statistical control.