New drugs are tested for safety. The absence of any toxic effect is taken to be evidence. When a security investigation reveals no instances of improper conduct, it would be wrong to conclude that the investigation left us ignorant. … Professors Copi and Burgess-Jackson suggest that the following factors be kept in mind: (a) How many individuals are attempting the proof; (b) whether those individuals are experts, or at least competent, on the matter at hand; (c) whether those attempting the proof are motivated (have an incentive) to prove the matter; (d) whether those attempting the proof have the technology and other resources necessary to conduct the inquiry and make the proof; (e) for how long a time the individuals have tried to prove the proposition.

Ruggero J. Aldisert, Agatha D. Aldisert, Logic for Lawyers- A Guide …, loc. 1079