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## Table 3.5: Likelihood of the Effect Standards – ATO Operations and NAS Equipment 

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The values in Table 3.5 are derived from an analysis of historical ATC data mapped to the established engineering standard (the current version of Advisory Circular 25.1309-1, System Design Analysis ) and can be applied to both ATC and Flight Procedures. The ratios binding each expected occurrence rate range were determined through calculations made using ten years of aviation data. In each calculation, the numerator was the number of occurrences of a given severity level occurring during a ten-year period, as obtained from various relevant databases. The denominator was the number of ATO operations (or flight hours) in that ten-year period, as obtained through the Operations Network database or the National Transportation Safety Board database. The value was adjusted to reflect a forecasted air traffic increase. A cut-off point of 10 -14  was established to define the boundaries of credible events for the purposes of calculating likelihood. Figure 3.6 depicts the likelihood continuum and the expected occurrence rate ranges. 

## 3.5.4.3.4 Determining Likelihood When No Data Are Available 

For some NAS changes, the necessary data are not available. There may not be a similar enough change/procedure/situation in the NAS to provide similar data from which to estimate a rate of occurrence. In situations where modeling is not feasible, pure subject matter expertise is the only input available, providing a qualitative approach to determining likelihood. This approach is only recommended when all avenues of data collection have been exhausted or when the change proponent is attempting to implement a new operation for which no data exist. For a majority of changes to the NAS, SMEs can collect and analyze data from a similar NAS change to determine the number of expected occurrences of an effect. 

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