When do you know that your measures have successfully mitigated the identified risk? If the control procedures achieve a "clinically acceptable" risk of harm to a patient, then your mitigation strategies are effective. Be aware that you can minimize risk, but it will never be entirely eliminated, hence, a "clinically acceptable" risk. Even the best processes are only 99.9% successful, as random error may occur. What is important is that if a failure does occur, you have a mechanism within your Quality Control Plan to quickly pinpoint where the failure occurred in the process and if the risk is unacceptable. If it is unacceptable, risk control procedures must be revised to further reduce the risk.