The Rule of Three

https://en.wikipedia.org/wiki/Rule_of_three_(statistics)

The rule of three states that if a certain event did not occur in a sample with n subjects, the interval from 0 to 3/n is a 95% confidence interval for the rate of occurrences in the population. When n is greater than 30, this is a good approximation of results from more sensitive tests.

Eypasch, Ernst, et al. “Probability of adverse events that have not yet occurred: a statistical reminder.” Bmj 311.7005 (1995): 619-620.