Hmnnn…

Is this the same as what drives the different methods/calculations for control charting  of  variable (measured) vs. attribute (counts) data, eg shouldnt attribute p, np, c or u c control charts be used for daily yield/defect rates (counts) rather than variable spc control charts?

this might help u…
http://www.thepdfshop.co.uk/ppm/html/reflow.htm

http://www.isixsigma.com/library/content/c021202a.asp
http://www.qualitydigest.com/may/spctool.html
http://www.i-q-i.com/whento.htm