scope:

Description of Driving Event:

Specifications are written to control the baseline requirements to which the Space Station is designed, constructed, verified, and delivered. A common practice within most specifications is to reference documents (know as applicable documents) with the specification as a source of requirements. Early in the development of the specifications the undesirable practice of "circular" referencing of documents infiltrated the specifications within the Space Station Program. Circular references is, in general, a condition by which a specification or document at a low level of the specification (or document) tree hierarchy references a specification of document at a higher level in the specification (or document) tree hierarchy. The following example illustrates the impact of "circular referencing."

Within the Space Station Program the top level specification is the System Specification (SSP 41000). SSP 41000 references numerous documents. Among the referenced documents in SSP 41000 are SSP 30558 and SSP 30559. The SSP 30558 and SSP 30559 documents, in turn reference SSP 41000 (completing the "circle"). Other specifications within the Space Station Specification "Tree" referenced SSP 30558 and SSP 30559. Because 41000 is an applicable document in 30558 and 30559, a lower level specification with the spec tree (for example, the United States Lab (USL) Spec) was placed in the posture of inadvertently being subjected to all requirements within the System Specification. There are two undesirable results caused by this practice:

1. Obviously the USL was not intended to fulfill all requirements as defined in the System Specification. The USL, however, under this structuring could have been construed as needing to meet all System Spec Requirements.

2. Maintenance of the "applicable document" section of each individual specification becomes tangled and interwoven to the point that nonapplicable requirements must be verified. For example a modification to SSP 30559 would result in a required document change of every specification within the Space Station Specification Tree.

Root cause: Failure to establish guidelines that define the purpose, use, and relationships of applicable documents. (Center Data Manager's note: The confusion over applicable documents arises quite often from an author's perceived need to justify or establish authority for a document versus identifying documents that, when cited as applicable, will aid in document implementation. The two purposes are quite different but in a poorly defined process both can be considered as meaning "applicable.")