The DPMO-Chart is also referred to as the Number Defects per Million Opportunities chart. For a sample subgroup, the number of times a defect occurs is measured and plotted as a value normalized to defects per million opportunities. Similar to a c-chart, the data values are always plotted as if a subgroup sample size equivalent to one million defect opportunities was used. Since usually actual sample subgroups of one million are not used, the defect data is scaled upward by a factor of \(\left[\frac{1,000,000}{(defect opportunities per unit) * (actual sample size)}\right]\). In that fashion a defect rate of 2 out of a sample size of 100, with 4 defect opportunities per unit, becomes an DPMO defect rate of 5,000 (\( 2 * \left[\frac{1,000,000}{4*100}\right] = 5,000\)). Since the plotted value is normalized to a fixed sample subgroup size, the size of the sample group can vary without rendering the chart useless
Note that this chart tracks the number of defects, not the number of defective parts as done in the p-chart, and np-chart. For any give part, you can have 0 to N defects. Defects are things like scratches, dents, chips, paint flaws, etc.; think of the last car you bought. Also, a defect does not indicate any magnitude of defect (such as might be measured in one of the variable control charts), only that it is, or is not a defect.
DPMO-Chart – 1
Defect data = { 2, 3, 8, 1, 1, 4, 1, 4, 5, 1, 8, 2, 4, 3, 4, 1, 8, 3, 7, 4 };