There is no “Three Mile Island” event coming for software

In Critical Digital Services: An Under-Studied Safety-Critical Domain, John Allspaw asks:

Critical digital services has yet to experience its “Three-Mile Island” event. Is
such an accident necessary for the domain to take human performance seriously? Or can it translate what other domains have learned and make productive use of
those lessons to inform how work is done and risk is anticipated for the future?

I don’t think the software world will ever experience such an event.

The effect of TMI

The Three Mile Island accident (TMI) is notable, not because of the immediate impact on human lives, but because of the profound effect it had on the field of safety science.

Before TMI, the prevailing theories of accidents was that they were because of issues like mechanical failures (e.g., bridge collapse, boiler explosion), unsafe operator practices, and mixing up physical controls (e.g., switch that lowers the landing gear looks similar to switch that lowers the flaps).

But TMI was different. It’s not that the operators were doing the wrong things, but rather that they did the right things based on their understanding of what was happening, but their understanding of what was happening, which was based on the information that they were getting from their instruments, didn’t match reality. As a result, the actions that they took contributed to the incident, even though they did what they were supposed to do. (For more on this, I recommend watching Richard Cook’s excellent lecture: It all started at TMI, 1979).

TMI led to a kind of Cambrian explosion of research into human error and its role in accidents. This is the beginning of the era where you see work from researchers such as Charles Perrow, Jens Rasmussen, James Reason, Don Norman, David Woods, and Erik Hollnagel.

Why there won’t be a software TMI

TMI was significant because it was an event that could not be explained using existing theories. I don’t think any such event will happen in a software system, because I think that every complex software system failure can be “explained”, even if the resulting explanation is lousy. No matter what the software failure looks like, someone will always be able to identify a “root cause”, and propose a solution (more automation, better procedures). I don’t think a complex software failure is capable of creating TMI style cognitive dissonance in our industry: we’re, unfortunately, too good at explaining away failures without making any changes to our priors.

We’ll continue to have Therac-25s, Knight Capitals, Air France 447s, 737 Maxs, 911 outages, Rogers outages, and Tesla autopilot deaths. Some of them will cause enormous loss of human life, and will result in legislative responses. But no such accident will compel the software industry to, as Allspaw puts it, take human performance seriously.

Our only hope is that the software industry eventually learns the lessons that the safety science learned from the original TMI.

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