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Observation Interference - The category of test interference that consists of output functions and/or failure modes that are 1) neither upstream nor downstream from functions in a test's coverage, 2) upstream from the test location, and 3) not themselves included in the test coverage. Compare with Creation Interference and  Propagation Interference.
In eXpress, Observation Interference most frequently comes into play for signature tests whose symmetry type has been set to Signature Identifies Operation. In the sample diagram below, where the coverage for a Signature Identifies Operation test performed at OUT-1 consists of function B, the Observation Interference for this test would consist of function E (the only function that is neither upstream nor  downstream from B, yet is upstream from OUT-1).