Because processes will randomly fluctuate over time and will often deteriorate in the long run, Motorola introduced the practice of reducing their calculated sigma values by 1.5 standard deviations to provide a more conservative estimate of the long term performance of the process.
Use: If we failed to account for the random fluctuations over time, the probability of realizing a defect (for every one million opportunities for a defect), we would be living under a false sense of security. Performing at exactly six sigma process equates to a probability of two (2) defects per BILLION. Normally distributed processes will fluctuate up to 1.5 sigma (either side of the mean) over time; therefore, after we account for that 1.5 sigma shift, the probability for a defect per 1 million opportunities turns out to be 3.4 DPMO (Defects per Million Opportunities).
