Resilience Engineering #6: Color Inside The Lines! – The Barrier Model

by Gary Monti on July 12, 2011

The barrier model for explaining failure is a quantum jump in improvement over the domino model. It helps get beyond two huge weaknesses with the domino model:

  • Focus only on the individual and individual behaviors, and;
  • Ignoring environmental factors that could play into the failures

Most of us are familiar with the barrier model in one form or another. While it is more robust than the domino model it is still essentially linear in nature. (In later blogs we’ll look at more dynamic approaches that go beyond linear thinking.)

The barrier model is an analogue from epidemiology. Think of how an infection spreads from country to country. There are latent conditions, e.g., no quarantine policies and procedures at customs, fast and easy air travel, and excellent street transportation within a city or country. This sets the stage for an actual infection incident to be a trigger event, which allows the infection to spread like wildfire. Concerns regarding avian flu a few years ago comprise a good example.

The central idea for the barrier model is following and staying within defined policies and procedures will trap pending failures and allow successful execution of a process, project, or any other activity.

The picture above is from one of my workshops and shows typical barriers.  A mistake occurs by slipping through a hole in the barrier rather than being deflected away. Again, this model is linear in nature and flows from the more general or strategic (the BLUNT END) and moves towards a specific action (the SHARP END), where the failure is visible and the actual damage occurs.

Specifically in this case, the first barrier is having the correct, integrated policies and procedures appropriate for the situation and is followed by the two remaining barriers of “training” and “task time.”

A mistake occurs when an action slips through a hole in the barriers. In this case a new hire is pushed into getting to work too rapidly. The first hole slipped through is lack of familiarity with policies and procedures (p & p) or the p & p may just be inadequate. The second hole slipped through is inadequate training. Finally, the third hole slipped through is not having enough time to get the job done. Notice how there is the assumption that given enough time with a given barrier there will be a compensation for failure at previous barriers and the pending mistake will be “deflected” or avoided. You can see where, over time, there could be a progressive over-reliance on successive, lower-in-the-ranks, closer-to-the-task managers compensating for less-than-ideal support coming from higher managers.

The barrier (or Swiss cheese) model is an obvious improvement over the domino model since a host of variables are taken into consideration, going beyond blaming the individual. This is a very common model and the familiarity referred to earlier is in being told, “Understand there is a way the organization gets things done. Learn it and stick to it. In other words, ‘Color inside the lines!’” With this attitude, though, there usually is a tinge of the domino model present because an error can be viewed as not sticking to what has been dictated.

The barrier frame of mind is usually fleshed out by having best practice groups put together to improve processes or there might be the formation of PMOs (project management offices) to standardize project methodologies, etc. This can work well in predictable situations where the rules work consistently. In such situations, a command-and-control structure can be established and as long as people stay within the limits set all will go well.

A closer look at the assumptions, though, will show limits to this model.

In complex or chaotic situations solutions are generated from the bottom up NOT the top down. Consequently, the barrier model and its linear approach fail to work in forecasting and preventing failures when complexity is present.

Another shortcoming of only using the barrier model with complex projects is complex projects are next to impossible to document. Ream after ream of paper can be consumed without getting to a clear, linear picture of the situation. Also, there is the fact that both the domino and barrier model focus on failure. But isn’t our goal to maximize success? Yes.

So, if the domino and barrier model are severely limited what does work? Tune in next week and see!

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