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.