Vicente does a nice job of illustrating the fact that system design isn't only about the construction of "stuff" -- it includes things like government legislation. The Walkerton example clearly demonstrated the different levels at which errors in the system can occur and how 'human-error' is only one small component of the system. Vicente identifies 3 political level system 'design levers' that influence systems: policy aims - that provide direction, legal regulations - penalties and rewards, and budget allocations -- decisions about where taxpayers' money should be spent. Vicente presents the Rasmussen Framework that aims to explain how accidents occur and how they can be prevented. A key feature in the framework is that decisions at the "higher levels should propagate down the hierarchy, whereas information about the current state of affairs should propagate up the hierarchy". As well, interdependencies across levels are critical to successful functioning of the system as a whole. Clearly these interdependences and propagation up and down the hierarchy were not present in the Walkerton case.
Rasmussen's Framework - Part 1
Another key point in this chapter is that "threats to safety or accidents are usually caused by multiple contributing factors -- relationships -- not just a single catastrophic decision or action" - a point which serves to emphasize the importance of management in the safety of complex technological systems.
The second level of Rasmussen's framework reveals how financial and psychological forces can "create behaviour gradients that cause work practices to migrate systematically toward the boundary of safety". It is interesting to note that this 'migration' doesn't usually have any immediate, visible, or negative impact. It may be years, as in the Walkerton example, before the impact of decisions (migrations) are felt. Vicente equates the "sum total of uncoordinated attempts at adapting" to "preparing the stage for an accident".
Rasmussen's Part II
The phrase that sums it all up for me in this chapter is: "Accidents in complex technological systems don't usually occur because of an unusual action or an entirely new, one-time threat to safety. Instead, they result from a systematically induced migration in work practices combined with an odd event or coincidence that winds up revealing the degradation in safety that has been steadily increasing all the while" (p. 276).
A short powerpoint movie with key points from chapter 9
- it goes fast, so be prepared to hit the pause button.
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