When comparing process complexity versus process capability, the two remain interrelated.
The extent to which processes remain capable, that is fall within the defined statistical limit, depends on the extent to which project managers can control variability. Process complexity is a significant factor in the process remaining within the set limits, without the variability exceeding such limits. Simple processes are easy to control and manage. When processes become more complex, it becomes more error-prone, more difficult to understand, more difficult to maintain, and liable for frequent shutdowns.
Understanding process complexity allows predicting process capability, or the extent or number of times the process fulfills customer requirements. Improving process capability, or bringing the process under statistical control in turn, requires making fundamental changes to the process, making it less complex, easier to understand and control by removing the sources of variability.
One popular manifestation of the synthesis of process complexity and process capability is Lean Six Sigma. Lean approaches process complexity by eliminating waste and streamlining the process and the Six Sigma Methodology ensures the process attains statistical compliance, ensuring process capability.