The more elbow room a process has, the more confident we can be that, as long as it remains in control, it will not produce any non-conforming output. Conversely, we can be fairly sure that a process which has less than 6 sigma units of elbow room will be incapable of consistently meeting the specifications. The diagram below shows one way in which a process can be incapable.

However, we cannot be certain that for any given process where the specified tolerance spans more than 6 sigma units, the output will all fall within the specification limits. Measuring the elbow room takes no account of where the process average is in relation to the specification limits.

Consider a process for which the elbow room is 8 sigma units. If the process is centred on the target value, then there will be 4 sigma units on either side of the process average.

If, however, the process is centred some way off target, say 2 sigma units below the nominal value, then some fraction of the output will fall below the lower specification limit.