Excellent reproducibility of functionality evaluation of welding in thick-plate tailored blank processes was obtained by using a standard S/N ratio calculated on the assumption that the characteristics of parent materials were in their standard and ideal states. The reproducibility obtained by using the standard S/N ratio also proved to be superior to the reproducibility obtained by using a zero-point proportional expression with Hooke's law considered as the ideal function, as has been done for joining problems in the past.