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Metrics
Metrics contains the logic that is used to store the output from all computes - consequences, risk, and project performance. Project performance is calculated for the default threshold and any additional thresholds.
Consequences and risk are stored in a ConsequencesDistributionResult object.
Project performance consists of annual exceedance probability, long-term exceedance probability, and conditional non-exceedance probability (assurance) for an impact area scenario compute. A project performance object stores a histogram of annual exceedance probabilities, and several histograms of stages.
Annual exceedance probability is computed as the realization of the probability of exceedance of the threshold stage on the sampled stage-frequency function. The histogram of annual exceedance probabilities can be used to compute the mean and median annual exceedance probabilities and the assurance of an annual exceedance probability.
Assurance of a threshold (conditional non-exceedance probability) is calculated for each of the standard recurrence intervals (.9, .98, .99, .996, .998) based on a histogram of stages realized at each recurrence interval for each iteration of the compute. The CDF of the threshold stage for each histogram are the estimates of the conditional non-exceedance probability.
Long-term exceedance probability uses the mean annual exceedance probability and a defined number of years N to describe the probability of the threshold being exceeded at least once in N years: 1-(1-AEP)^N.
Annual exceedance probability and conditional non-exceedance probability is calculated differently for an impact area scenario with a levee.
Annual exceedance probability is calculated as the sum of the products of the incremental exceedance probability and average failure probability for a small range of stages. For default levees, the probability of failure is zero until the top of the levee at which point the probability of failure is 100%.
Assurance (conditional non-exceedance probability) is calculated similarly to annual exceedance probability. For a very small range of stages, the product of the incremental exceedance probability and failure probability at the average of the stages in the very small range is calculated. These products are summed from the bottom of the system response curve to the stage at which the probability of levee failure is 100%.
A threshold is the value for which project performance is calculated. The threshold is an exterior stage. The default threshold is the exterior stage at which a user defined percent of the damage occurs at the user defined recurrence interval. The default threshold default values are 5% of the 1% AEP event.