Figure 83-13 James Reason's model of accident causation. Latent failures at the managerial level may combine with psychological precursors and event triggers at the operational level to initiate an accident sequence. Most accident sequences are trapped at one or more layers of the system's defenses. The unforeseen combination of organizational or performance failures with latent errors and triggers may lead to a breach of the system's defenses allowing the accident to happen. The diagram should be envisioned as being 3-dimensional and dynamic—with "shields" moving around and holes in the defense's opening and closing.^[320] (Redrawn from Reason JT: Human Error. Cambridge, Cambridge University Press, 1990.)