ABSTRACT
A comprehensive discussion related to the effect of variability on the reliability of products can be found in (Haugen, 1980, Carter, 1997, Booker, et al., 2001, Fowlkes and Creveling, 1995).
Depending on the source, the same component of the same material manufactured to the same specification is usually characterised by different properties. Between sources, variation exists even if the variations of property values characterising the individual sources are very small. Furthermore, due to the inherent variability of the manufacturing processes, even items produced by the same manufacturer are usually characterised by different properties. Because of the natural variation of critical design parameters, early-life failures are often due to unfavourable combinations of values (e.g. worst case tolerance stacks) rather than due to a single production defect.
