> If there's a constant small probability of injury per item packed, then of course doing twice as much work per work day, will lead to twice as many injuries per employee per work day
This assumes injury-risk is a linear function of number of items packed. That's not true. Fatigue increases injury-risk - that is how tired you and your entire team is while performing the task. If the person sorting the packages up the line is tired and making mistakes, the person binning it is more prone to mistakes, leading to an injury. In summary, there is a "fatigue-threshold" after which injury-risk increases non-linearly (say exponentially) with every item packed.
This assumes injury-risk is a linear function of number of items packed. That's not true. Fatigue increases injury-risk - that is how tired you and your entire team is while performing the task. If the person sorting the packages up the line is tired and making mistakes, the person binning it is more prone to mistakes, leading to an injury. In summary, there is a "fatigue-threshold" after which injury-risk increases non-linearly (say exponentially) with every item packed.