Imagine that time to complete almost all tickets is 4 hours. But one of those tickets will have an unforeseen problem, increasing the time by -4 hours (instant solve) to a year. The outlier now dominates the schedule. (my summary of:
https://erikbern.com/2019/04/15/why-software-projects-take-longer-than-you-think-a-statistical-model.html
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April 17, 2019