The likelihood rating in the risk register is a measure of how probable it is that a risk event will occur, given the current conditions and controls. The risk practitioner should change the likelihood rating if there is a significant change in the effectiveness of the controls that are implemented to prevent or reduce the risk. For example, if a control becomes obsolete, ineffective, or bypassed, the likelihood rating should increase, as the risk event becomes more likely to happen. Conversely, if a control becomes more efficient, reliable, or robust, the likelihood rating should decrease, as the risk event becomes less likely to happen. The other options are not likely to cause a change in the likelihood rating, as they are not directly related to the probability of the risk event. Risk appetite is the amount of risk that an organization is willing to accept in pursuit of its objectives. Control cost is the amount of resources that are required to implement and maintain a control. Risk tolerance is the acceptable level of variation that an organization is willing to allow for a risk to deviate from its desired level or expected outcome. These factors may influence the risk response or the risk acceptance, but not the likelihood rating. References = Risk and Information Systems Control Study Manual, Chapter 1:
IT Risk Identification, Section 1.4: Risk Register, p. 25-26.