Which statement about the process capability ratio is NOT true?

• A. The process capability ratio is computed as the difference of the upper and lower specification limits divided by six standard deviations.
• B. The process capability ratio is a ratio of the specification to the process variation.
• C. A capable process has a process capability ratio less than one.
• D. The process capability ratio is a ratio for determining whether a process meets design specifications.

Answer: (C)

The statement regarding the process capability ratio that is not true is the one indicating that a capable process has a process capability ratio less than one. The process capability ratio (Cp) is a measure used to assess how well a process can produce output within specified limits. For a process to be considered capable, the ratio must be equal to or greater than one. This indicates that the process variations (typically measured as six standard deviations) are within the specified limits, meaning that the process can meet customer requirements.

In contrast, if the process capability ratio is less than one, it signals that the process is not capable, as the variability exceeds the specifications. This is essential for quality control and ensuring that products meet design specifications effectively. Thus, the understanding of process capability is vital for businesses aiming to maintain quality standards and improve operational efficiency.