A term of art in the audit profession that is tossed around but not well-understood is “reasonable assurance.” What does it actually mean when an audit report attests with reasonable assurance that the financial reporting or internal controls are reliable?

Accountants, auditors, their clients, and even regulators generally acknowledge that it is impossible to assert with absolute certainty that an event will or will not occur or even whether or not it has already occurred.

All professions accept that some form of judgment is involved when rendering an opinion, and that those judgments aren’t always perfectly accurate. In the U.S. criminal justice system, for example, a very high burden of proof—“beyond a reasonable doubt”—is required to convict an individual, yet glaring mistakes still occur under this system. Even in the world of science, where research, carefully controlled experiments, and peer-reviewed results are expected to lead to a closer approximation of truth, presumptions are in continual flux and can be upended by new evidence.