Why is there a 1.5 sigma shift between long-term quality data and the reported short-term sigma quality level in the Six Sigma methodology?

	a.	The 1.5 sigma shift forces processes to actually have less than 1 defect per million opportunities to be a 6 sigma process.
	b.	The 1.5 sigma shift is an allowance given in some cases where defects are hard to measure.
	c.	The 1.5 sigma shift accounts for the tendency of processes to drift around the mean over the long term, and removes the effect of this drift.
	d.	The 1.5 sigma shift represents the difference between measurement of automated processes vs. human-driven manual processes.