Proving an absence

An empty result has three causes, not two: the thing is absent; the probe is broken; or it is there but out of reach — spelled differently, generated at build time, outside the paths searched. The output distinguishes none of them. Diagnostic PCR runs an internal control for this: without one a negative can mean no target was present, "but it could also mean that the reaction was inhibited."

Absence of evidence is evidence of absence — the biconditional falls out of the probability axioms (Sober) — but only as strong as the chance the probe would have hit. Establishing that chance cheaply is the job.

When to use

• About to write "X does not exist", "nothing implements Y", "no test covers Z".
• Reproducing a reported failure — failing to reproduce is an absence claim.
• Auditing a plan or catalog for stale or unbuilt claims.
• Stating any number, or reviewing a finding that reports something absent.