Confusing Safety with Reliability

Assumption 1 Safety is increased by increasing system or component reliability. If components or systems do not fail, then accidents will not occur. 

This assumption is one of the most pervasive in engineering and other fields. Hie problem is that it s not true. Safety and reliability are different properties. One does not imply nor require the other A system can be reliable but unsafe. It can also be safe but unreliable. In some cases, these two properties even conflict, that is, making the system safer may decrease reliability and enhancing reliability may decrease safety. The confusion on this point is exemplified by the primary focus on failure events in most accident and incident analysis. Some researchers in organizational aspects of safety also make this mistake by suggesting that high reliability organizations will be safe [107,175,177,205,206]. 

Attributed to Will Rogers (e.g.. New York Times, 10/7/84, p. B4), Mark Twain, and Josh Billings (Oxford Dictionary of Quotations, 1979, p. 49), among others. 

Because this assumption about the equivalence between safety and reliability is so widely held, the distinction between these two properties needs to be carefully considered. First, let s consider accidents where none of the system components fail. 

The Mars Polar Lander loss is a component interaction accident. Such acddents arise in the interactions among system components (electromechanical, digital, human, and social) rather than in the failure of individual components. In contrast, the other main type of accident, a component failure accident, results from component failures, including the possibility of multiple and cascading failures. In component failure accidents, the failures are usually treated as random phenomena. In component interaction accidents, there may be no failures and the system design errors giving rise to unsafe behavior are not random events. 

---

Although they are often confused, reliability and safety are different properties. The pilots may reliably execute the landing procedures on a plane or at an airport in which those procedures are unsafe. A gun when discharged out on a desert with no other humans or animals for hundreds of miles may be both safe and reliable. When discharged in a crowded mall, the reliability will not have changed, but the safety most assuredly has. 

It is primarily a rehabihty technique. Many failure modes listed will have no safety concern, and much effort may be spend in obtaining or justdying the failure rates (step 2a(ii)) of these. So, be aware of confusing reliability objectives with safety objectives. 

Verbal and non-verbal communication govern our personal relationship with the outside world - with everybody else. Until a reliable form of telepathy is developed, whatever message we want to send or receive to others has to be by word of mouth directly, written, or by gesture and appearance - involving the five senses. Sometimes there can be conflicts between the forms of the message. A verbal instruction from a foreman to wear safety shoes, when the foreman is not doing so himself, causes confusion. 

---