Requisite variety refers to the capacity required to overcome disturbances the system is “likely” to encounter. When an automated system is overwhelmed, human operators must come to its aid. We might say that the automated system lacked variety, and the humans increased the variety of the combined system. Deciding how much variety to include is a design decision. Are we designing for a hundred-year flood or a magnitude-8 earthquake? We weigh the likelihood of the disturbance against the cost of including the variety required to resist it.