That's apparently based on Motorola's data regarding process drift.
> Motorola has determined, through years of process and data collection, that processes vary and drift over time – what they call the Long-Term Dynamic Mean Variation. This variation typically falls between 1.4 and 1.6.
> Motorola has determined, through years of process and data collection, that processes vary and drift over time – what they call the Long-Term Dynamic Mean Variation. This variation typically falls between 1.4 and 1.6.
https://www.isixsigma.com/new-to-six-sigma/dmaic/15-sigma-pr...