Work out outbreak detection threshold instantly with clear inputs, formula shown and shareable results.
Sets an outbreak threshold at the historical mean plus a chosen number of standard deviations and compares the current count against it.
Mean plus 2SD threshold
Threshold = historical mean + k x historical standard deviation; z = (current cases - mean) / standard deviation
Under a normal approximation it flags roughly the top 2.5% of periods, a workable false alarm rate for routine surveillance.
Poisson or negative binomial methods perform better than a normal approximation when counts are small.