The widespread use of heuristics, or rules of thumb in engineering require judgement to use properly. Writer B V Koen defines a heuristic as anything that provides a plausible aid or direction in the solution of a problem but is in the final analysis unjustified, incapable of justification and fallible. Heuristics are used in the absence of better knowledge or as a short-cut method of working out something that would be too expensive or time consuming to work out more scientifically. Yield strength is a characteristic of a material that indicates when it will break or fail. It is therefore an approximation but does not guarantee a correct answer but gives the engineer an approximation to work with.
Engineers also make use of engineering handbooks in their design.
For example a structural engineer will use handbooks to choose the size and shape of a beam that will be able to take the stresses he or she has calculated the beam will have to function under. Handbooks enable engineers to choose materials to suit their purposes without conducting the experiments themselves necessary to verify that choice. Yet handbooks only cover a certain range of conditions and situations and the person using the handbook needs to know what limitations are implied. Even then these handbooks disguise a wide variation in materials which the engineer needs to be aware of. Figure 2 shows the huge variation in measurements of thermal conductivity of copper at different temperatures found by a number of researchers.