The capability ratio is the inverse of the Cp index. Remember that the Cp index is the specification spread divided by the process spread of six standard deviations. Flip that formula around and you would be dividing the process spread by the specification spread. In the case of a capability ratio, you would be looking for a smaller-is-better characteristic. With the Cp index, you are looking for a larger-is-better characteristic.
Use: Sometimes it’s easier for the sake of understanding to use the capability ratio because it tells you what percentage of the specification window that the process variation is consuming. For instance, if you have a Cp of 1.33, the specification window is consuming 133% of the process variation. That’s kind of difficult to visualize. Using the same data, the Cp of 1.33 would equate to a capability ratio of 0.75 (flip the formula). Now you go to your boss and say that the process variation is consuming 75% the specification window. That is easier to visualize. That’s easier to understand. There’s nothing wrong with using either index as long as you understand that with the Cp index it is a larger-is-better characteristic and with a capability ratio it is a smaller-is-better characteristic.
