What makes a bowtie diagram?
The Speed Read
A Bowtie Diagram shows how you manage a key risk in a single easily understood picture. This article outlines the main elements that you need to include.
The headline risk or Top Event at the centre is the result of a hazard. There may be many possible reasons why this hazard becomes an incident (see triggers, on the left). After an incident there may be multiple unwanted outcomes (on the right). The rest of the diagram illustrates barriers which reduce the chance of an incident. And, if it does happen, how to mitigate or reduce the severity of the outcomes.
Hazard: A Hazard is a potential source of harm, at the root of a bowtie diagram. It could be thought of as ‘an accident waiting to happen’, but an accident is something much more specific.
Top Event: When control of the hazard is lost, things start to happen. But nothing bad has happened, yet!
Trigger: An action or event which could cause loss of control of the hazard (the Top Event). The frequency of the trigger is important. Is it happening all the time, or is it common, or very rare? Choose a number between -6 (incredibly rare) and 0 (all the time) to represent this. The more negative the number is, the better because it means further apart.
Outcome: outcomes or consequences are the bad things which might happen after the Top Event has occurred. They might be health-affecting or financial, legal or just consequential in some way. Rank them by Severity with Least Concern scoring -6 and Worst Case scoring 0. The more negative the number is, the better because it means less severe.
Control: controls are what get in between a Trigger and the Top Event, or if the Top Event occurs, then they help to reduce the severity. Controls are rated between 0 (not much use) and -2 (good). They reduce the frequency of a Trigger, or they reduce the severity of an Outcome by simple addition.
Controls are also identified by Control Types, which can be used to identify personal roles, or something more general. Relying on people to follow processes is usually important, over-reliance on a small group can introduce new risks.
Video: Scoring and Escalations in Bowtie Diagrams
Escalation: Things which affect the usefulness (or reliability) of a control are known as Escalations or degradations, because they make a situation more serious. Examples of an Escalation might be untrained inexperienced personnel, or patchy maintenance. If you add an Escalation to a Control, it will negate its effectiveness, initially. The example Escalation below is “Unaware of Guidance”
Escalation Control: In order to remove the degradation effect of an Escalation, add more controls to it. For example, an induction process for new recruits, or maintenance audits. The example Escalation Control is “Release Procedure includes checks”
Note: Because there are only so many words that you can fit into a diagram, we added Notes. Notes allow you to add additional information to a control, or hyperlinks to relevant documents.