Entering PRISM Software
When PRISM is selected as the calculation model, selecting a part category of Software and a subcategory of PRISM Software causes parameters to appear in the Prediction Data pane. The PRISM parameters for a software component are very different from those for an electronic hardware component. Additionally, process grades are not used to adjust the failure rate calculated for a software component as they are for a hardware component.
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PRISM and 217Plus are the only calculation models with component models for software. When one of these models is selected, you can use its failure rate equation for a software component to calculate the predicted failure rate. Otherwise, you can use the general software reliability equation from The Rome Laboratory Reliability Engineer's Toolkit. For more information, see General Software Model.
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The following table describes the part parameters required for a PRISM software component. The default values are for when little information is available because the software component is assumed to be new and not widely deployed. As the software component becomes more widely deployed, more accurate values should be entered for Fault Latency and Fault Activation.
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Parameter
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Type
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Description
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Growth After Deployment
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Checkbox
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Indicates whether reliability growth processes are in place to ensure that the software component undergoes further modifications after deployment so that it grows in reliability. When this checkbox is cleared (default), Months Since, Months To, and Fault Density, Stable are unavailable because no reliability growth processes are in place. When this checkbox is selected, these fields are made available since reliability growth processes are in place.
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Deployment
The parameters under this heading supply information about the deployment of the software.
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Months Since
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Real
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The number of months since the software was released. This property is available only when Growth After Deployment is selected. The default is 12.00.
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Lines of Code (x1000)
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Integer
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The number of lines of code in the software. The unit is thousands of lines of code. Thus, 10 would be entered if there were 10,000 lines of code. The default is 20.
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Fault Density, Initial
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Real
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The number of faults per thousand lines of code at the software’s initial release. The default is 6.00.
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Fault Latency
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Real
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The average number of times a fault occurs before the fault is corrected. The default is 2.00.
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Stabilization
The parameters under this heading supply information about code maturity. If the deployment time is greater than the stabilization time, the stabilization time is used for the failure rate calculation. The calculated failure rate for the software never decreases after the stabilization time.
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Months To
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Real
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The number of months until code maturity (or fault stabilization) is reached. The default is 48.00, which is standard for the initial release. If this is a subsequent release, the value in this field should be changed. The standard for subsequent releases is 24.00. The property is available only when Growth After Deployment is selected.
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Fault Activation
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Real
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The percentage of modules exhibiting the occurrence of faults. The default is 100.00.
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Fault Density, Stable
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Real
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The proposed or actual number of faults (depending on how long since the release of the software), per thousand lines of code. The default is 10.00. The property is available only when Growth After Deployment is selected.
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Average Severity
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Real
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The percentage of faults that is disruptive or critical to the custom. The default is 50.00.
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[Derating Parameters]
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For more information, see Derating Parameters.
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