In understanding how to address abnormal situations, its important to understand the factors that cause or influence abnormal situations. Root and contributing causes may appear in isolation or in combination with each other. However, in most cases , the abnormal situation appears as a result of the interaction among multiple sources. For example, a frequent plant practice is to push the process to its limits to maximize production. In purposely pushing the limits of the process, the probability of the equipment failing, the process getting out of sync, and/or personnel making errors greatly increases; the equipment's original design limits are challenged, the process is operating at or beyond its original design constraints, and personnel are being asked to monitor and interact with a process that is complex and reaching the limits of their cognitive and physical response capabilities. At any point in the process, one or more of these factors may contribute to the onset and escalation of an abnormal state. The resulting abnormal situations vary in their complexity and effect on the process.

There are three principal types of sources or causes of abnormal situations:

  • People and work context factors
  • Equipment factors
  • Process factors

A fourth source, physical environmental antecedents (e.g., lightning, earthquakes, storms), will not be discussed here because of their infrequent occurrence and usually obvious role as a root cause.

The three sources of abnormal situations were identified in the review of member companies incident reports. The average percentages for each type of source are shown in the figure below. These data were obtained in the initial ASM site studies from 1992-3 incident reports. Of all the incidents reports, we selected only those events that had an impact on process operations.



The average percentages shown had the following:

  • People and work Context Factors: 35% - 58%
  • Equipment Factors: 30% - 45%
  • Process Factors: 3% - 35%


The data from the incident reports should be interpreted with some caution given that the findings are biased in a number of ways. First, individuals at some sites indicated a reluctance to identify people as the source of an incident. Second, the data are based on a small number of sites and thus, reflects idiosyncrasies of the sites sampled. We present the ranges above to give an indication of the variability across the sites. Despite these biases, we did find these averages to be consistent with the literature that has summarized the sources or root causes of a large number of incidents. (Butikofer,1990; Lorenzo,1990).

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The frequency distribution above shows the relative contribution of root cause categories. Each unit in the figure represents 3 incidents. Within the People and Work Context Factors, the largest contributors were inadequate or no procedure (16/59, 27%), inadequate or incorrect action (14/59, 24%) and fail to follow procedure/instruction (14/59, 24%). Within the Equipment Factors, the largest contributor was equipment or mechanical failure (24/50 or 48%). Within the Process Factors, the largest contributor was operating beyond the original process design limits (21/30 or 70%).