The

rule of threestates 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.

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

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