I can think of many examples of products that are one-off and have mostly subjective acceptance criteria. In fact, just about anything that involves a lot of creativity falls into this category.
Examples include:
Custom architecture
Design of anything where appearance is a prime criteria (e.g.; couture clothing or a customized car interior)
Fine arts
All of these are cases where the product is normally one of a kind and the success or failure of the product is mostly subjective, indeed differenct people will have different perceptions of whether the product met its acceptance criteria. Moreover, what was successful in a previous similar product may not be successful in the product being prepared. The one-off product being prepared will give you one subjective data point, with no guarantee that data point can be applied to other one-off products. Accordingly, applying Six Sigma methodology to meeting these subjective requirements is pretty much doomed to failure because you cannot legitimately generate the data required to back up success/failure claims. It’s all a matter of “taste”.
Mind you, all of these products have some acceptance criteria that can be objectively measured. A building must meet Code requirements for fire exits and such, clothing (usually) needs to cover defined naughty bits, a car will have to hold a specified payload, and a commssioned fine arts project (e.g.; a statue) may have to meet dimensional restraints. Here you can use Six Sigma methodology, because you can objectively demonstrate if something does or does not meet a specific objective requirement, and make corrections as required. Moreover, this information can be transferred to the next product with the same objective requirement.
I trust this explains my position.
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