In his book Quality is Free, Philip Crosby put forward the notion that setting any target for an acceptable number of defects in a process was a plan to fail. Our target should be zero. A marksman, knowing he’s unlikely to hit the target dead-centre doesn’t aim off-centre (other than to adjust for ‘scope settings, range, wind, etc) – no, he’ll still aim for the centre of the target.
There is a story that a large American car manufacturer ordered a consignment of a component from a new Japanese supplier, partly to see if that country’s reputation for quality was justified. To test them, they placed a much tighter AQL than on their usual suppliers (one they already struggled to meet). [AQL = Acceptable Quality Level – a term used to define the percentage of defects that would be allowed within a batch.] When the consignment arrived, there were two boxes – a large one with the requisite number of components, all to specification, and a smaller box containing a handful of extra ones. When asked why the extra box, the supplier replied that it contained the number of defective components that had been ordered – and that they had been made specially for the order. Unlike the local supplier who was happy to ship defective components along with the good, the Japanese supplier would only accept shipping out the good – manufacturing and shipping defective parts was failure.
It makes sense – why aim for anything less than getting it right first time, every time? Six-sigma follows a similar line, although recognising perfection may not be achievable (either in reality or at an economic cost) aiming to reduce the defect level almost to insignificance (and before somebody argues that six-sigma levels are below a particular significance level, that’s not the same). Even then, occasional defects are acceptable within the system; target zero says none are acceptable.88These two concepts aren’t incompatible if considered together as part of a larger system. For example, we need to recognise that there will be defects in an aircraft but we strive to ensure we manage any that could have fatal consequences.