Active factors are the final events or circumstances which led to an occurrence. Their effect is often immediate because they occur either directly in the system's defence (e.g., disabled warning system) or the site of the productive activities (i.e., the integrated activities of the work system's liveware, software and hardware elements), which would indirectly result in the breaching of the system's defence (e.g., use of the wrong procedure).
Underlying factors may reside at both the personal and the organizational levels; they may be present in the conditions that exist within a given work system (referring to the preconditions element in the model). Examples of underlying factors include inadequate regulations, inadequate procedures, insufficient training, high workload and undue time pressure.
In practice, steps 1 and 2 may not be mutually exclusive. As the investigator begins the data collection step, it would be only natural that an attempt be made to place the information, albeit often fragmentary in the preliminary stages of an investigation, into the context of an occurrence sequence. To facilitate this concurrent activity, the SHEL and Reason models can be combined as illustrated in figure 2.
