Six sigma: That would be 3.4 defects per 1,000,000 opportunities (DPMO)....
...it is applied as a key quality metric based upon discrete (attribute) data and the results and consequences (e.g., frequency/severity) of failure of the production unit or system.
Today, however.....for the rest of us, the majority of our business operations and service delivery platforms consist largely of continuous data which does not, and likely should not, require classic six sigma quality metrics and systems. The transformation of six sigma, even lean six sigma, into our nonparametric socially networked enterprise operating system is where today's organizations and project management (our collective "we") get into trouble.
The majority of businesses today speak, hear and spend, but never see continuous LSS transformed to PM & operations. There exist tools which can identify and adapt value registered as six sigma; but for operations to achieve the 3.4/1,000,000 locus on the bell curve...we must regard the six sigma business case I discuss in the blog page of this site.
Below is the basic LSS DMAIC (5 step process) toolkit for qualifying and driving quality governance today. Still, with DMAIC as a quality approach there remains the caveat that the use and implementation be predicated upon Deming's 14 Points for Quality Management (1986). But that is another story.