Normal Accident and HRO theorists agreed that interactive complexity and tight coupling can, theoretically, lead to a system accident. However, they hold different opinions on whether those system accidents are inevitable or are manageable. Serious accidents in high risk, hazardous operations can be prevented through a combination of organizational design, culture, management, and human choice. Theorists of both schools place a lot of emphasis on human interaction with the system as either cause (NAT) or prevention (HRO) of a systems accident. High reliability organization theory and HROs are often contrasted against Charles Perrow's Normal Accident Theory (NAT) (see Sagan for a comparison of HRO and NAT). NAT represents Perrow's attempt to translate his understanding of the disaster at Three Mile Island nuclear facility into a more general formulation of accidents and disasters. Perrow's 1984 book also included chapters on petrochemical plants, aviation accidents, naval accidents, "earth-based system" accidents (dam breaks, earthquakes), and "exotic" accidents (genetic engineering, military operations, and space flight). At Three Mile Island the technology was tightly coupled due to time-dependent processes, invariant sequences, and limited slack. The events that spread through this technology were invisible concatenations that were impossible to anticipate and cascaded in an interactively complex manner. Perrow hypothesized that regardless of the effectiveness of management and operations, accidents in systems that are characterized by tight coupling and interactive complexity will be normal or inevitable as they often cannot be foreseen or prevented. This pessimistic view, described by some theorists as unashamedly technologically deterministic, contrasts with the more optimistic view of HRO proponents, who argued that high-risk, high-hazard organizations can function safely despite the hazards of complex systems. Despite their differences, NAT and high reliability organization theory share a focus on the social and organizational underpinnings of system safety and accident causation/prevention.As research continued, a body of knowledge emerged based on the studying a variety of organizations. For example, fire incident command system, Loma Linda Hospital's Pediatric Intensive Care Unit, and the California Independent System Operator were all studied as examples of HROs.