The reliability Attribute is used for assigning reliability values to elements in a design. The editing dialog for reliability Attributes (depicted below), allows the Analyst to define the reliability value in terms of either failure rate or MTBF (mean time between failures).This dialog can be invoked by clicking on one of the "Edit Default" buttons on the Attribute Definitions dialog (the same editing fields appear on the right side of the Attributes panel when editing the value of a reliability Attribute for a specific element in the design). Note: the reliability Attribute used to define the failure rates for eXpress analyses must be assigned the name "Reliability".
The Analyst can enter reliability data as either a failure rate (FR) ormean time between failures (MTBF). The two are generally considered to be interchangable, since (with failure rates specified as failures per million hours) MTBF = 1,000,000 / FR.
Failure Rate - Following common practice, eXpress assumes that all failure rates are expressed as failures per million hours. When failure rates are provided using a different, non-time basis (failures per mission, failures per 100 rounds, etc.), these values should first be converted into failures per million hours so that they can be compared with other, time-based failure rates. If not, the Analyst must recognize that all time-based calculations will only be meaningful relative to each other (for example, it can be determined which ambiguitygroups will be isolated most frequently, but not the expected time between failures). If failure rates are entered into eXpress based on failures over time, but assume different units (e.g., failures per thousand hours or failures per million flight hours), it is important that all reliability values in the system use the same units. Finally, when individual failure rates are based on operating times of the specific elements (as opposed to system usage time), then the duty cycles associated with different elements must be accounted for either by scaling failure rates prior to entry, or by the use of duty cycle lists.
MTBF - Enter the time between failures. Because a time Attribute is used for the MTBF, this value can be entered using any of the units supported within time Attributes. As with failure rates, when individual MTBFs are based on operating times of the specific elements, then the numbers should either be scaled prior to entry (to account for the different duty cycles associated with each element) or duty cycle lists should be set up within the model so that eXpress can automatically scale reliability data.
Regardless of which method is used, the other field will be calculated using the formula MTBF = 1,000,000 / FR (the non-selected field is not updated during entry, so the Analyst will have to change Attributes or elements and then come back in order to see the updated field).
