By the time the control phase is reached, several improvements may have been made, and revisions may have been required until sustainable progress can be achieved within the previous improvement phase. The new, improved process is then monitored over a pre-determined period to verify improvements and performance. The final step is to create and establish a monitoring or control system handed off to the process owner to assist in sustaining and maintaining the new, improved process. The objective of the control phase is to ensure that the improved process can now deliver the critical variables to the process and stay within control limits, known as the specification range. The established upper and lower specification limits, or USL and LSL, define the process specification range.
