Process capability refers to the ability of a process to produce output that meets customer specifications consistently. It compares the voice of the process (its natural variation) with the voice

Here's an explanation of process capability and its indicators:

Process capability refers to the ability of a process to produce output that meets customer specifications consistently. It compares the voice of the process (its natural variation) with the voice of the customer (specification limits).

1. Indicators which show a process is capable:
A process is considered capable when its natural variation is significantly smaller than the allowable specification limits. Key indicators include:

• Process Capability Index ($C_p$): This measures the potential capability of a process, assuming it is centered between the specification limits. A $C_p$ value of $\ge 1.0$ generally indicates that the process is capable of fitting within the specification limits. A value of $\ge 1.33$ is often preferred for robust processes.

$$C_p = \frac{\text{Upper Specification Limit (USL)} - \text{Lower Specification Limit (LSL)}}{6\sigma}$$

• Process Performance Index ($C_{pk}$): This measures the actual capability of a process, taking into account whether the process mean is centered. It is a more practical indicator as it considers both spread and centering. A $C_{pk}$ value of $\ge 1.0$ indicates that the process is capable and its mean is sufficiently far from the nearest specification limit. A value of $\ge 1.33$ is often a target for good performance.

$$C_{pk} = \min\left(\frac{\text{USL} - \mu}{3\sigma}, \frac{\mu - \text{LSL}}{3\sigma}\right)$$

• Process output within specification limits: Visually, if nearly all (e.g., 99.73% for a 3-sigma process) of the process output falls within the Upper and Lower Specification Limits, the process is capable.

2. Indicators which show a process is not capable:
A process is not capable when its natural variation is too wide or its mean is shifted, causing output to fall outside the specification limits. Key indicators include:

• $C_p$ or $C_{pk}$ values less than 1.0: This indicates that the process variation is too large relative to the specification limits, or the process mean is too close to a specification limit, leading to a high probability of producing defects.
• Process output exceeding specification limits: If a significant number of products or services produced by the process fall outside the USL or LSL, the process is not capable of meeting customer requirements.
• High defect rates: A high percentage of non-conforming products or services is a direct indicator of an incapable process.