Model Selection
As indicated in Model Descriptions, you typically choose the calculation model to use for a prediction based on the parts in your system and system requirements. Oftentimes, a particular model is more widely accepted in a particular industry. For instance, if you are in the telecommunications industry, you are likely to use a Telcordia, PRISM, HRD5, IEC TR 62380, or RDF 2000 model.
The top-most assembly in the System Tree Items table represents the entire system. In this assembly’s Calculation Data pane, a model is always selected for Calculation model. For more information, see Calculation Data Pane. If MIL-HDBK-217 is licensed, MIL-HDBK-217 FN2 is the default for the top-most assembly in a newly created System file. In this field, you can select any model that is licensed. While your selection for the top-most assembly is not shown in the Calculation Data pane for lower-level assemblies, they inherit this selection.
In most cases, you can mix models, which means that you can choose the model by which to calculate failure rates on an assembly-by-assembly basis. When you select a model for Calculation model for a lower-level assembly, this model overrides the selection that it inherited from its parent assembly. All of its child assemblies then inherit this selection.
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The FIDES, IEC TR 62380, and RDF 2000 models calculate failure rates very differently than other models. FIDES models use mission profiles instead of mission time as the basis of their calculations, and they have their own special set of support files for adjusting base failure rates. The IEC TR 62380 and RDF 2000 models use the cycling profiles and applicable phases supplied in Prediction Cycling Profile files for their failure rate calculations. Because of how differently these models calculate failure rates, they cannot be mixed with other models or use methods supplied by other models. You can select one of these models for Calculation model for only the top-most assembly. When this is this case, any model overrides specified for lower-level assemblies are ignored.
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For example, for the top-most assembly in a newly created System file, assume that you selected Telcordia Issue 3 for Calculation model. As you insert assemblies beneath it, they inherit this model. If you select MIL-HDBK-217 FN2 for Calculation model for a lower-level assembly, it is the model for this assembly and the model inherited by its child assemblies. When mixing models is supported, you can specify an override model at any assembly level or at the part level. For a part, you select the override model for Calculation model in its General Data pane. For more information, see General Data Pane (Part).
Additionally, when mixing models is supported, you can use methods from one model to adjust failure rates calculated by another model. For instance, you can use Telcordia calculation methods to adjust the failure rates calculated by other models. Or, when 217Plus or PRISM is licensed, you can use the process grades, Bayesian analysis, and predecessor analysis methods supplied with these models to adjust the failure rates calculated by other models.
Regardless of model selections, the process for calculating prediction results is the same. Once results are calculated, in both the System Tree Items table and Parts Table, any part that is not supported by the selected model is shown in magenta to indicate that no failure rate has been calculated for it. As described in Unsupported Parts, you can either specify a failure rate in the Prediction Data pane for this part or select a different model for Calculation model in its General Data pane. This ability to mix models and methods provides you great flexibility in finding ways to obtain accurate predictions.