ComplianceFebruary 07, 2019

3 pillars to define barrier effectiveness

Many bowtie creators are struggling to find out what makes a barrier effective.

The barriers are mapped in a bowtie, but how are they performing in the real world?

Barrier effectiveness is often defined based on expert judgment and historical experiences. In this blog, we provide other insights into barrier effectiveness by describing 3 pillars to help you find out what the effectiveness is of your barriers so that your bowtie analysis will be improved.

  • Activities
  • Adequacy vs reliability
  • Accountability

Barrier-supporting activities

Once you have added all elements (top event, hazard, threats, consequences, escalation factors and (escalation factor) barriers) to the bowtie and feel like you are finished, you are actually just starting. What you have just created was the paper world; the purpose of bowties is to understand the real world. Thus, we must close the gap between these two worlds.

Let’s take an easy example: the seatbelt as a barrier. As a manager of a delivery company that uses vans, you might think that every van has a seatbelt installed and every driver is using it. However, do you have a ‘regular inspection and maintenance’ activity? Is there a seatbelt alarm installed to force people to use the seatbelt? Are people aware of the consequences of not wearing a seatbelt?

Ask the same questions about your barriers and take the next step by making sure to add check activities to your bowtie. Activities support the integrity of the barrier and thus strengthen the barrier. Barrier supporting activities exist in different formats, think about training, certification, maintenance, inspection, and design qualifications and so on.

Adequacy versus reliability

To understand what barrier effectiveness is, we look at adequacy and reliability of the barrier. Adequacy is determined in combination with the threat size. Barriers can occur on multiple threat lines. This does not mean the barrier has the same effectiveness on each line because threats have different adequacies.

Let’s look at the example below. Defensive driving is a barrier for both threats, but for slippery road conditions the effectiveness is higher. Because slippery road conditions is a much more stable threat than unexpected maneuvers, the barrier defensive driving is considered to be more adequate. An unexpected maneuver has much more deviation and it is hard to train drivers to all sorts of unexpected maneuvers, therefore it is considered as a less effective barrier.

When we look at the reliability of the barriers, we consider the escalation factors, incident data and other sources. Seatbelts are considered as less effective because you rely on the person to wear it. An airbag, however, will usually expand during a collision. Therefore, we consider an airbag as a much more reliable barrier compared to the seatbelt. 


Assigning accountabilities to barriers is one of the major advantages of bowties, especially when it comes to communication. Bowties are a great communication tool because of its clear layout.
It is important to assign accountabilities to barriers because if a barrier does not have someone accountable it can be ignored or forgotten. Barrier-supporting activities also need an accountable person.

Take your bowtie to the work floor and make sure everybody understands his or her responsibilities.

Define the effectiveness of your barriers

Now, we have looked at three aspects of barrier effectiveness that hopefully gives you more insights and helps you define the effectiveness of barriers. It is important to look at a barrier in a very detailed level because all these details tell you a lot about barrier effectiveness. The last practical tip: Do not only look at the examples we’ve given in this blog because there are so many more!

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