Although there is some uncertainty and ambiguity in defining safety culture, there is no uncertainty over the relevance or significance of the concept. Mearns et al. stated that "safety culture is an important concept that forms the environment within which individual safety attitudes develop and persist and safety behaviours are promoted". With every major disaster, considerable resources are allocated to identify factors that might have contributed to the outcome of the event. Consideration of the considerable detail revealed by inquiries into such disasters is invaluable in identifying generic factors that "make organisations vulnerable to failures" From such inquiries, a pattern emerges; organizational accidents are not a result of randomly coinciding "operator error", chance environmental or technical failures alone. Rather, the disasters are a result of a breakdown in the organization's policies and procedures that were established to deal with safety, and the breakdown flows from inadequate attention being paid to safety issues. In the UK, investigations into incidents such as the sinking of the MS Herald of Free Enterprise passenger ferry (Sheen, 1987), the Kings Cross underground station fire (1987) and the Piper Alpha oil platform explosion (1988) raised awareness of the effect of organisational, managerial and human factors on safety outcomes, and the decisive effect of 'safety culture' on those factors. In the US, similar issues were found to underlie the Space Shuttle Challenger disaster, subsequent investigation of which identified that cultural issues had influenced numerous "flawed" decisions on behalf of NASA and Thiokol management that had contributed to the disaster. The lesson drawn from the UK disasters was that, "It is essential to create a corporate atmosphere or culture in which safety is understood to be and is accepted as, the number one priority."