105 disaggregated control questions can be analysed using Boolean logic to derive a measure of control reliability but in other cases this is not possible. Reliability of mitigation controls can sometimes be derived from verification data in the same way that the reliability of prevention controls can be derived. However, for other mitigation controls this can be challenging if the mitigation control requires the unwanted event to occur for the reliability of the control to be determined. For example, reliability of emergency response to attend to injured persons should be assessed from practice not from plans. For these more challenging mitigation controls, simulated activities can be used to verify controls (e.g. emergency response drills). In addition, incident information can be used to improve the understanding of the reliability of mitigation controls if investigations assess the effectiveness of these types of controls as part of the investigation process. Figure 3 – Determination of control effectiveness (Source: Hassall & Harris, 2017) The determination of the efficacy of controls can vary across controls and operations. Some controls have 100% efficacy at mitigating fatalities. Examples include guarding and barricading that remove people from line-of-fire zone have 100% efficacy (noting that the reliability may vary). Other controls will have variable efficacy depending on the nature of the event. Examples include berms and airbags which can prevent fatalities in some incidents but Control effectiveness
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