Telcordia Issue 4, Telcordia Issue 3, and Issue 2 support the calculation of mean failure rates and standard deviations for individual parts. These results are stored in the following data fields, Failure Rate, Mean Failure Rate, and Standard Deviation, which you can insert in the System Tree Items table and Parts Table. The mean failure rates and standard deviations of lower-level items are rolled up to obtain mean failure rates and standard deviations for assemblies.

The values for Failure Rate are not rolled up. The part failure rates for an assembly do not add up to the assembly failure rate. This fields stores the confidence failure rate for all System Tree levels (parts, assemblies, and system). While earlier Telcordia and Bellcore models assume an upper confidence interval of 90%, Telcordia Issues 4, 3, and 2 allow you to specify the upper confidence interval. If a percentage value is not supplied for Confidence Interval under Prediction in the System file properties, the 90% default is used. For more information, see Prediction System File Properties.

For models prior to Telcordia Issue 2, the values for Failure Rate, Mean are the same as the values for Failure Rate, and the values for Failure Rate, Standard Deviation are zeros.

When calculating the standard deviation, all parts are considered to be independent. Telcordia Issues 4, 3, and 2 group all parts of the same type together, calculate the mean failure rate and standard deviation, and then multiply by the quantity. As a result, these later Telcordia models would calculate two independent parts differently than a part with a quantity of two. To avoid this, all parts are considered to be independent. Two separate parts having the same category/subcategory combination with the same calculation data calculate the same as a similar part with a specified quantity of two.