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3.5.4.3 Determining Likelihood 

3.5.4.3.1 Likelihood versus Frequency 

Likelihood is defined as the estimated probability or frequency, in quantitative or qualitative terms, of a hazard’s effect or outcome. More specifically, the concept of likelihood can be separated into two components: likelihood/probability and frequency. Frequency  is how often a given effect occurs; it is a known value determined (for example) by monitoring a hazard and its effects to identify initial, current, or residual risk (see Section 4.3.1 and Section 4.3.4). Conversely, likelihood is an expression of the probability of a hazard’s effects occurring (i.e., a rate of how often a given effect is expected to occur), which is used to estimate initial and predicted residual risk. Provide a rationale for likelihood estimations in the HAW. 

3.5.4.3.2 What to Consider When Defining Likelihood 

## Frequency and Modeling 

Frequency is sometimes used to help estimate likelihood, but historical data do not always represent future conditions. Historical frequency may be zero for a given procedure, but that does not mean that the future likelihood is also zero. For example, a facility may conduct a procedure that has unreported incidents that could lead to an undesirable outcome, such as a loss of separation or a collision. Likewise, a facility may not have encountered the scenario or system state that exposes the more severe outcome. Consider all potential effects that are derived from indicators of the operation in all credible scenarios. This practice is required to challenge the philosophy of, “It has not happened in the past, so it will not happen in the future.” 

When possible, use modeling to examine the effects of hazards that are too rare to have significant historical statistical data available. 2 If modeling is required and data are available, the risk assessment should be based on statistical or observational data (e.g., radar tracks). Where there are insufficient data to construct statistical risk assessments, input from Subject Matter Experts (SMEs) can be used. This means that if the true rate of a particular type of operation is unknown, it can be estimated using expert judgment. It is important to note that complex proposed NAS changes, such as changes to separation standards, require quantitative data to support the associated risk analysis. 

