Probability of Failure


The first step in determining the criticality of an asset is determining the likelihood or probability of failure. Entirely eliminating the possibility of failure is unrealistic and most likely impossible. However, systems can work to minimize failure, especially for critical assets that have high consequences of failure. There are four modes by which an asset can fail: mortality, financial inefficiency, capacity, and level of service. Each of these four failure modes has a variety of factors that contribute to that potential mode of failure. These factors can range from O&MOperations and Maintenance history to customer expectations to condition of the asset. There is not one factor that is the most common cause of failure, and these factors may impact green and gray assets differently. When thinking about how each individual asset might fail, all four mechanisms need to be considered. It is also worthwhile to consider which failure mode is most likely to affect that particular asset.


To calculate the criticality of an asset, the probability of failure of that asset needs to be quantified. Each asset should be assigned a probability of failure (POF) rating  based on how likely the asset is to fail. These ratings should be based on a probability of failure rating structure created by system staff. Each failure rating in the structure should have a general description of likelihood of occurrence. It is important to create these rating systems so all staff members can rate the probability of failure of a given asset similarly. The ratings should not be compared to other systems; this is meant as an internal tool only. The goal is for a system to determine which of their assets are more likely to fail than other assets in their system.

The probability of failure rating is used in the calculation of the criticality score of each asset. As a reminder, the criticality score (overall risk score) for an asset is the probability of failure rating multiplied by the consequence of failure rating. To learn more about consequence of failure and consequence of failure ratings click here. The criticality scores of assets will indicate their risk level and help system staff prioritize risk mitigation strategies including asset maintenance, replacement, rehabilitation or repair.