Residual risk is the risk that remains after the risk mitigation plans have been implemented. Residual risk reflects the effectiveness of the risk response in reducing the likelihood or impact of the risk. The best evidence that risk mitigation plans have been implemented effectively is the change in the level of residual risk. A change in the level of residual risk can be measured by comparing the risk level before and after the risk mitigation plans have been executed. A change in the level of residual risk can also be evaluated by comparing the actual residual risk with the target or acceptable residual risk. A change in the level of residual risk can demonstrate how well the risk mitigation plans have achieved the risk objectives and met the risk criteria. A change in the level of residual risk can also provide feedback and lessons learned for future risk management activities. References = Residual Risk: Definition, Formula & Management, Residual Risk: What It Is and How to Manage It, Residual Risk: How to Calculate and Manage It.