  Effective software safety standards will contribute to confidence, or
assurance, in the safety of the systems in which the software is used. It is
infeasible to demonstrate a correlation between standards and accidents, but
there is an alternative view that makes standards "testable". Software projects
are subject to uncertainty; good standards reduce uncertainty more than poor
ones. Similarly assurance or integrity levels in standards should define an
uncertainty gradient. The paper proposes an argument -based method of reasoning
about uncertainty that can be used as a basis for conducting experiments
(tests) to evaluate standards.
