Root Cause Analysis using Rothmans Causal Pies

rothmans causal pies

It sometimes seems to me that in the tech industry, maybe because we’re often playing with new technologies and innovating in our organisation, or even field, (when we’re not trying to pay down tech debt and keep legacy systems running), we’re sometimes guilty of not looking outside our sphere for better practices and new (or even old) ideas.

Whilst studying for my Master’s degree in Global Health, I discovered the concept of “Rothman’s Causal Pies”.

The Epidemiological Triad

Epidemiology is the study of why and how diseases (including non-communicable diseases) occur. As a field, it encompasses the entire realm of human existence, from environmental and biological aspect to heuristics and even economics. It’s a real exercise into Systems Thinking, which is kinda why I love it.

In epidemiology, there is a concept known as the “Epidemiological Triad”, which describes the necessary relationship between vector, host, and environment. When all three are present, the disease can occur. Without one or more of those three factors, the disease cannot occur. It’s a very simplistic but useful model. As we know, all models are wrong, but some are useful.

This concept is useful because through understanding this triad, it’s possible to identify an intervention to reduce the incidence of, or even eradicate, a disease, such as by changing something in the environment (say, by providing clean drinking water) or a vaccination programme (changing something about the host).

What the triad doesn’t provide, however, is a description of the various factors necessary for the disease to occur, and this is especially relevant to non-infectious disease, such as back pain, coronary heart disease, or a mental health problem. In these cases, there may be many different components, or causal factors. Some of these may be “necessary”, whilst some may contribute. There may be many difference combinations of causes that result in the disease.

To use heart disease as an example, the component causes, or “risk factors” could include poor diet, little or no exercise, genetic predisposition, smoking, alcohol, and many more. No single component is sufficient to cause the disease, and one (genetic predisposition, for example) may be necessary in all cases.

Rothman, in 1976, came up with a model that demonstrates the multifactorial nature of causation.

Rothman’s Causal Pies

An individual factor that contributes to cause disease is shown as a piece of a pie, like the triangles in the game Trivial Pursuit. After all the pieces of a pie fall into place, the pie is complete, and disease occurs.

The individual factors are called component causes. The complete pie, which is termed a causal pathway, is called a sufficient cause. A disease may have more than one sufficient cause, with each sufficient cause being composed of several component causes that may or may not overlap. A component that appears in every single pie or pathway is called a necessary cause, because without it, disease does not occur. An example of this is the role that genetic factors play in haemophilia in humans – haemophilia will not occur without a specific gene defect, but the gene defect is not believed to be sufficient in isolation to cause the disease.

An example: Note in the image below that component cause A is a necessary cause because it appears in every pie.

Root Cause Analysis

I’m a huge proponent of holding regular retrospectives (for incidents, failures, successes, and simply at regular intervals), but it seems that in technology, particularly when we’re carrying out a Root Cause Analysis due to an incident, there’s a tendency to assume one single “root cause” – the smoking gun that caused the problem.

We may tend towards assuming that once we’ve found this necessary cause, we’re finished. And whilst that’s certainly a useful exercise, it’s important to recognise that there are other component causes and there may be more than one sufficient cause.

The Five Why’s model is a great example of this – it fails to probe into other component factors, and only looks for a single root cause. As any resilience engineer will tell you: There is no Root Cause.

The 5 whys takes the team down a single linear path, and will certainly find a root cause, but leaves the team blind to other potential component or sufficient causes – and even worse: it leads the team to believe that they’ve identified the problem. In the worst case scenario, a team may identify “human error” as a root cause, which could re-affirm a faulty, overly-simplistic world view and result in not only the wrong cause identified, but harm the team’s ability to carry out RCAs in the future.

Read more about the flaws in the “five whys” model in John Allspaw’s “Infinite Hows”.

In reality, we’re dealing with complex, maybe even chaotic, systems, alongside human interactions. There exist multiple causal factors, some necessary for the “incident” to have occurred, and some simply component causes that together become sufficient – the completed pie!

Take Away: There is usually more than one causal pie.

An improved approach could be to use Ishikawa diagrams, but in my experience, when dealing with complex systems, these diagrams very quickly become visibly cluttered and complex, which makes them hard to use. Additionally, because each “fish bone” is treated as a separate pathway, interrelationships between causes may not be identified.

Instead of a complex fishbone diagram, try identifying all the component causes, and visually complete (on a whiteboard for example) all the pies that could (or did) result in the outcome. You almost certainly won’t identify all of them, but that doesn’t matter very much.

If we adopt the Rothman’s causal pie model instead of approaches such as the 5 whys or Ishikawa, it provides us with an easy to use and easy to visualise tool that can model not only “what caused this incident”, but “what factors, if present, could cause this incident to occur again?“. 

In order to prevent the incident (the disease, in epidemiological terms), the key factor we’re looking for is the “necessary cause” – component A in the pies diagram. But we’re also looking for the other component causes.

Application: The prevention of future incidents.

Suppose we can’t easily solve component A – maybe it’s a third party system that’s outside our control – but we can control causal components B and C which occur in every causal pie. If we control for those instead, it’s clear that we don’t need to worry about component A anyway!

Next time you’re carrying out a Root Cause Analysis or retrospective, try using Rothman’s Causal Pies, and please let me know how it goes.

Addendum: “Post-Mortem” exercises.

Even though the term “post-mortem” is ubiquitously used in the technology industry as a descriptor for analysis into root causes, I don’t like it.

Firstly, in the vast majority of tech incidents, nobody died – post-mortem literally means “after death”. It implies that a Very Bad Thing happened, but if we’re trying to hold constructive, open exercises where everyone present possesses enough psychological safety in order to contribute honestly and without fear, we should phrase the exercise in less morbid terms. The incident has already happened – we should treat it as a learning opportunity, not a punitive sounding exercise.

Secondly, we should run these root cause analysis exercises for successes, not just for failures. You don’t learn the secrets of a great marriage by studying divorce. The term “post-mortem” isn’t particularly appropriate for studying the root causes of successes.