Which statement about Cp/Cpk in process capability?

Which statement about Cp/Cpk in process capability?

• A. Cp measures actual performance; Cpk measures potential.
• B. Cp and Cpk are not used in aerospace.
• C. Cp/Cpk quantify potential and actual process capability and centering.
• D. Cp/Cpk measure only inventory levels.

Answer: (C)

The main idea here is that Cp and Cpk measure what a process can potentially do versus how it is actually performing, taking into account how centered the process is within the specification limits. Cp looks at the spread of the process relative to the tolerance window, assuming the process is perfectly centered. It’s a measure of potential capability: if the process were centered, how much of the spread would fit inside the specs? It uses the formula involving the process standard deviation and the width between the upper and lower spec limits. Cpk, on the other hand, factors in where the process mean actually sits relative to those limits. It looks at the distance from the mean to each limit, scaled by the process spread, and takes the smaller of the two distances. This gives the actual capability, showing not only the spread but also how well the process is centered. If the mean is exactly centered, Cp and Cpk would be similar; if the process is shifted away from center, Cpk will be smaller than Cp. So the statement that these indices quantify potential and actual process capability and centering captures the essence: Cp reflects what the process could do if it were perfectly centered (potential), while Cpk reflects what the process is doing in reality, including centering (actual capability). This is why it’s the best description. In contrast, they are not only about inventory levels, and they are widely used in aerospace manufacturing to assess process performance.

The main idea here is that Cp and Cpk measure what a process can potentially do versus how it is actually performing, taking into account how centered the process is within the specification limits. Cp looks at the spread of the process relative to the tolerance window, assuming the process is perfectly centered. It’s a measure of potential capability: if the process were centered, how much of the spread would fit inside the specs? It uses the formula involving the process standard deviation and the width between the upper and lower spec limits.

Cpk, on the other hand, factors in where the process mean actually sits relative to those limits. It looks at the distance from the mean to each limit, scaled by the process spread, and takes the smaller of the two distances. This gives the actual capability, showing not only the spread but also how well the process is centered. If the mean is exactly centered, Cp and Cpk would be similar; if the process is shifted away from center, Cpk will be smaller than Cp.

So the statement that these indices quantify potential and actual process capability and centering captures the essence: Cp reflects what the process could do if it were perfectly centered (potential), while Cpk reflects what the process is doing in reality, including centering (actual capability). This is why it’s the best description. In contrast, they are not only about inventory levels, and they are widely used in aerospace manufacturing to assess process performance.