Verifiability is a measure of how testable a statement or requirement can be. One of the jobs of a Test Engineer (TE) is to read requirements with an eye toward testability. The TE has to ask himself how a requirement could be tested. If an approach is not found then the requirement is marked "not testable". This can often be a problem with the grammar. Statements with several conjunctions like (AND) and (OR) can obscure the meaning of a requirement configuration such that more is implied than intended.

The TE is concerned with all possibilities of test configurations and sometimes the use of conjunctions implies more configurations than the writer wanted. This is often a result of sloppy writing. For Example; The rocket takes off, flies and lands in water. It may be intended that the rocket only lands in the water but the conjunctions imply flying through the water which might not be conveniently tested. If a requirement is determined to not be testable then it is rejected in a review so that the systems engineer can rework it as to be verifiable. You may think this is a bit nit-picky, however many system disappointments arise from needs with many conjunctions, misinterpreted to mean something other than what was desired. This cascades through a system design in ridiculous ways.

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