Standard control charts for monitoring the defect rate of a process include the c-chart and the u-chart. C-charts can be used to monitor the actual "counts" of defects from a process. When the number of items within a subgroup varies, the u-charts, which tracks the actual defects per unit, can be used. But, these two charts are not ideally suited for monitoring defects occurring on parts of varying complexities. Without another method, comparing the data of parts of different complexities would be akin to comparing apples and oranges.
DPMO (defects per million opportunities) and DPTO (defects per thousand opportunities) charts take into account the different complexities of parts to provide a more accurate view of your process. This is what makes DPMO and DPTO charts so valuable. DPMO charts allow one to specify the number of opportunities for a defect to occur on a part, product, subassembly or final assembly. When the number of defects for a subgroup is entered, the rate of defects per million opportunities is calculated for each subgroup using the formula found in Equation 1.
Equation 1 Calculation of Defects Per Million Opportunities (DPMO).
The DPMO formula normalizes the data on a common scale (in defects per million opportunities) and allows parts of varying complexities to be plotted on the same chart.
Identifying Defect Opportunities
Assume a company performs a final visual inspection test for the circuit boards they manufacture. For each of the finished circuit boards (parts) they plan to track on the DPMO chart, they must know how many defect opportunities exist.
Figure 1 Add Defect Opportunities Window With Opportunities/Piece Specified.
For example, the CB-0974-664 circuit board has 8 leads and 2 components. The leads must function properly and the components must be placed correctly on the substrate. Therefore, at final inspection, there are a total of 10 different opportunities for defects on the CB-0974-664 circuit board.
Obviously, for larger, more complex circuit boards, there will likely exist a greater number of defect opportunities per piece. Before creating a DPMO chart, first specify the number of defect opportunities for a part/test combination by clicking "Options>Defect Opportunities." This will bring up the "Add Defect Opportunities" window shown in Figure 1. In the "E. Opportunities/Piece" box, specify the number of potential defects. This must be done for each part/test data combination to be tracked on the DPMO chart.
The Right Type of Data
DPMO charts can only be created for "Defects" data. You may confirm the feature type specified for each test by using the ProFicient Database Manager utility (see Figure 2). If the feature type you wish to include on a DPMO chart is not defined as "Defects," you will need to re-create it. Once it has been specified, ProFicient does not allow the feature type to be edited or changed.
Figure 2 Feature Type for DPMO Tests must be "Defects."
Creating DPMO Control Charts
DPMO charts are created in the same way as any other chart within InfinityQS: in the menu bar, click Chart>New. and make the appropriate selections from the windows wizard. In the Chart Type window (see Figure 3), choose "DPMO Chart" from the "B.Graph 1" combo box. If the feature type for the test of interest has been correctly specified as "Defects" in the Add/Edit Test Data window in Database Manager, then users will be able to select from either a u-, c-, DPMO or DPTO chart.
Figure 3 Chart Type Window for Selecting Defects Control Chart Types
After the chart type selection has been performed, the resulting DPMO control chart might look like the chart found in Figure 4. Note that the DPMO chart includes the defects data from 3 different parts. To include data from more than one part on a single chart, alter the chart’s Data Selection by choosing Chart>Configuration>Data Selection and choosing multiple parts.
Figure 4 DPMO Control Chart Displaying Defects Data From Three Parts.
DPTO is short for "Defects Per Thousand Opportunities." The only difference in DPTO and DPMO charts is the scale. DPTO allows plotting of data in defects per thousand instead of million. To change a DPMO chart to a DPTO chart, click Chart>Configuration>Type and choose DPTO from the "B. Graph 1" combo box found in Figure 3.
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