The most helpful metric to measure the success of an internal audit activity in providing risk-based assurance is the percentage of highly significant risks covered by the internal audit plan. This demonstrates that the internal audit function is focusing its resources on the most critical areas that could impact the organization’s objectives, ensuring that significant risks are being addressed and managed appropriately. This alignment with the organization's risk profile is a key indicator of effective risk-based auditing. References: The IIA’s International Standards for the Professional Practice of Internal Auditing (Standards), specifically Standard 2010 - Planning, and Standard 2120 - Risk Management.