To understand Perrow's approach, one must first distinguish between what he calls incidents and accidents. Incidents and Accidents Perrow differentiates between simple incidents like backing a car into a telephone pole and nuclear accidents like Three Mile Island. Level one is the part, the smallest component of any system. Examples include valves, filters, gauges, etc. Level two is the unit, which is made up of parts.
Examples are motors, pumps, wiring panels, etc.
Level three is the subsystem, which is an array of units. Examples include propulsion systems on naval destroyers and navigation sets in aircraft. Level four is the system itself, which is the summation of its subsystems. Examples are aircraft carriers and space shuttles. Incidents and accidents both begin with equipment failure or human error, but accidents continue on out of control with multiple, unanticipated failures in units and subsystems.