The best key performance indicator (KPI) to measure the effectiveness of the security patching process is the percentage of patches installed successfully within the expected time frame. This KPI can help to evaluate how well the security patching process meets the predefined objectives and standards, and how timely the patches are applied to reduce the risk exposure. The percentage of patches installed by the security administration team, successfully during the first attempt, or without causing an unplanned system outage are other possible KPIs, but they are not as relevant as the percentage of patches installed successfully within the expected time frame. References = ISACA Certified in Risk and Information Systems Control (CRISC) Certification Exam Question and Answers, question 7; CRISC Review Manual, 6th Edition, page 202.