Quality Control Techniques for "Zero Defects"

Everyone is being exposed to the "zero defects" philosophy which establishes zero as a goal. This will not be achieved overnight but approached over time by continually striving to reduce targets. What kind of techniques are needed to assure zero defects? What constitutes an out-of-control situation? An attributes control chart conveys little information at or near zero defects. Assuring zero defects through sampling inspection leads to infinite samples or 100 percent inspection, assuming 100 percent inspection efficiency (the latter rarely exists, and efficiency probably gets worse at lower defect levels). Obviously, some new approaches to quality control (QC) techniques will be necessary at zero defects. One old standby is the variables control chart, overline{X} and R, but with the specification at least five standard deviations from the average. Thus one route to zero defects is a properly chosen specification. However if attributes data must be used, the standard p and u charts are not very useful. Perhaps a control chart that plots the number of good items between defects on a logarithmic scale to accommodate large numbers can be used, establishing upper and lower limits on the number of items between defects. Another problem area at zero defects is sampling inspection to assure targets. Following the approach above on good items between defects, the number of accepted lots between rejected lots can be a criterion. Sample sizes can be related to lot sizes as in MIL-STD-105D. If rejected lots come too close together, a procedure can be established requiring process shutdown until the problems are resolved. This will help promote the "making it right the first time" philosophy necessary to achieve zero defects.