Figure 2 shows the huge variation in measurements of thermal conductivity of copper at different temperatures found by a number of researchers. Engineers usually cope with the variability in materials and also their inability to predict exactly the stresses and strains their technologies will have to cope with whilst in use by applying a factor of safety . Engineer Barry McMahon has found his clients believe that a factor of safety implies certainty plus a bit more and they are far more concerned with the risk of conservative design than they are with other sources of risk. The factor of safety is itself a heuristic which changes with time and circumstance. For a factor of safety to be effective the means of failure must be known and the cause of the failure determinable by experiment.
All engineering structures incorporate factors of safety and yet some still fail.
When this happens the factor of safety might be increased. However when a particular type of structure has been used often and without failure there is a tendency for engineers to suspect that these structures are overdesigned and that the factor of safety can be reduced. Petroski comments The dynamics of raising the factor of safety in the wake of accidents and lowering it in the absense of accidents can clearly lead to cyclic occurrences of structural failures. He points out that this cyclic behaviour occurred with suspension bridges following the failure of the Tacoma Narrows Bridge. This process of fine-tuning or cutting safety margins to reduce costs in the face of success is not confined to structural engineering and is present across all engineering disciplines.