Step 4: Determine Underlying Causes
Each of the facts - actions, decisions and conditions - established in the previous steps is an opportunity to ask, "why?" Testing and analyzing the theories the team develops is an iterative process during which you discover important new facts and possible explanations.
Referring to the sequence of events diagram, ask "why" repeatedly until you identify the unsafe conditions, acts or procedures that contributed to the incident. Write down the answers. As you ask and answer these questions, you will see other questions. Remember to explore details, even when they aren't obviously key facts.
Answers to the initial series of "whys" provide clues about immediate causes. Continuing to probe and ask "why" again will lead investigators to discover the root or underlying causes - the more fundamental circumstances that caused or contributed to the crash.
Immediate causes - unsafe or substandard acts, practices or conditions that lead directly to the incident. These include things like driving a vehicle with worn out tires or while knowingly impaired by stress or fatigue, lack of concentration, speeding, not knowing or failing to follow a safe work procedure, etc. Immediate causes are symptoms of the underlying causes of the incident.
Root or Underlying causes - explain why the immediate causes occurred; they are the organizational circumstances that allow unsafe conditions to exist, the conditions that facilitate unsafe decisions and the fundamental reasons behind unsafe actions. They include inadequate work planning, unrealistic work demands, incomplete systems, incorrect procedures and many others.