Barriers are not created equal. Some are better than others. Barrier effectiveness is a way to assess how well a barrier performs. Effectiveness is often used as a single property of a Barrier. However, in this article we’ll break effectiveness down into two main elements. Adequacy and reliability.
If you look at defensive driving as a barrier, it actually features on multiple Threats. However, defensive driving is not equally effective for those Threats. That’s because the adequacy is different. Adequacy tells you to what extent a properly functioning Barrier will interrupt a particular scenario. It’s important to understand that adequacy is not an absolute measure. The adequacy of a barrier can be different depending on the scenario that it is controlling. This is also the main reason why you shouldn’t copy paste Barriers with an effectiveness rating. It could be that the effectiveness is different, because the adequacy is different.
Having a perfectly adequate barrier is not enough, it needs to actually work when needed. That’s what reliability is about. Will my barrier do what it’s supposed to do, when I need it? Assessing the reliability is done by looking at the Escalation factors (although not all Escalation factors necessarily impact the reliability), incidents in which the barrier failed or was missing, audit results and other sources.
For example, the barrier “Wearing a seatbelt” has an Escalation factor, which reduces the reliability, so we need to adjust the effectiveness of wearing a seatbelt to medium (indicated by the yellow color).
For more information on other dimensions of effectiveness which are not discussed here, read this excellent article by Snorre Sklet, which should be obligatory reading for every Bowtie aficionado anyhow:
Sklet, S. (2006). Safety barriers: Definition, classification, and performance. Journal of Loss Prevention in the Process Industries 19 (5) p. 494-506
You can download it here, the article in question starts at page 50.