Perrow's definitions can be confusing. A truck tire could blow, cause a wreck and kill a soldier. An extremely serious situation, but one he would categorize as an incident. Integral to his analysis are the concepts of interaction and coupling. The concept of interaction helps Perrow identify which systems are most prone to accidents.
Linear interactions describe highly structured systems which are logical, sequential and planned.
They function as a series of expected events in a predictable sequence. If damage to a part occurs, the problem can be identified and corrected with little disturbance to the overall system. Linear interactions are also characterized by minimal feedback loops which make it easier to understand and monitor the entire system. Complex interactions on the other hand, are less predictable. Unexpected events may occur, regardless of intended system designs.