My Problem with PIGs

You can't swing a euphemism without hitting one when you're playing in the risk management metaphor. They're everywhere. Whenever you start looking at anything risk management related, you are sure to find a PIG. PIG stands for Probability-Impact Graph - otherwise known as likelihood-consequence matrix or frequency-severity chart or some combination of these words. I'm most familiar with the LxC matrix label, so I'll use it from here on in.

Over the past year or so, I've being growing more and more uneasy with the application of this tool within the aviation safety environment. I wasn't seeing, however, the same discontent in others and therefore, started to doubt my own reservations. Luckily, I found some like-minded people over at LinkedIn (membership to both LinkedIn and the group are required to view the actual discussion) with a Mr Stephen Cresswell putting his thoughts on paper here.

My new best friends have identified a range of issues with the PIG, some of which apply to other applications and some of which are very similar to my concerns.

So what are my concerns?

The first one is to what do I apply the score - do I apply it to the hazard, the event or the outcome? For me, the outcome always seemed wrong because the consequence is contained within its definition thus it negates the need for that dimension of the score. The event gives you good opportunity to attach a likelihood of it occurring but what about an event with a variety of possible consequences or causes (hazards)? And for hazards, is it likelihood of existence or some consequential event and here we go wrapping ourselves up in knots.

Example time: Let's have some evil birds hitting lovely, peaceful planes1. On an airport, birds tend to cause a bit of stress in their operator's lives. How does one risk assess this problem?

Do you calculate the likelihood & consequence of the bird-strike event? Seems simple enough but how to you account for different birds in different areas affecting different phases of the aircraft's flight? Do you then apply the calculation to each bird species? How do you distribute this score across the possible outcomes?

And that brings me to my second beef with PIGs - risk is not a discrete score.

If risk is indeed a combination of likelihood and consequence, in the aviation safety context, I don't see how it cannot be expressed as a discrete score. The risk of a bird-strike is a continuum. Most of the time, i.e. high likelihood, the consequence will be minor or negligible (near-miss or small collision). Some of the time, i.e. lower likelihood, the consequence will be major (something broken) and on rare occasions, i.e. really low likelihood, you'll get a major media event.

So what do you score? The most likely consequence, the worst case the scenario, the most credible outcome, etc. etc. etc.?

For my last point, I'll steal directly from Mr Cresswell:

PIGs take a simplistic view of risk in which there is no system-based thinking about the relationships and dependencies between risks, opportunities and uncertainties.

Aviation is an extremely complex socio-technical system - it's the relationships that matter. Treating each "risk" as a separate line item with its own discrete score doesn't mesh with our thinking in other areas - especially, accident causation theory and the overall safety management system concept.

I'm going to try to develop these ideas over the coming weeks (with more regularity than to date) - stay tuned.

1. Last year I posted this on bird-strike risk management. I even used a PIG approach at the more strategic level but dropped it for the specie-specific risk assessment, instead I opted for a completely different approach.