Six Sigma

'The performance of a product is determined by how much margin exists between the design requirement of its characteristics (and those of its parts/steps), and the actual value of those characteristics. These characteristics are produced by processes in the factory, and at the suppliers.

Each process attempts to reproduce its characteristics identically from unit to unit, but within each process some variation occurs.

A variation of the process is measured in Std. Dev, (Sigma) from the Mean. The normal variation, defined as process width, is +/-3 Sigma about the mean.

Approximately 2700 parts per million parts/steps will fall outside the normal variation of +/- 3 Sigma. This, by itself, does not appear disconcerting. However, when we build a product containing 1200 parts/steps, we can expect 3.24 defects per unit (1200 x .0027), on average. This would result in a rolled yield of less than 4%, which means fewer than 4 units out of every 100 would go through the entire manufacturing process without a defect.

Thus, we can see that for a product to be built virtually defect-free, it must be designed to accept characteristics which are significantly more than +/- 3 sigma away from the mean.