Four common measures of process performance are Defects Per Unit (DPU), Defects per Million Opportunities (DPMO), Parts per Million Defective (PPM), and the Rolled Throughput Yield (RTY). The key to understanding the difference between these terms is to understand the difference between a “defect” and a “defective” item:
- A defect refers to a flaw or discrepancy on an item where more than one flaw (defect) can be found. For • example, a hospital admission form contains several fields of information that can be missing or incorrect, so a given form can have more than one defect. This means that a sample of 10 forms can show more than 10 defects.
- An item is said to be defective when the decision is made that the item is not acceptable, based either • on one characteristic or the accumulation of multiple defects. This means that a sample of 10 items can show a maximum 10 defective units.
Defects per unit (DPU) – the average number of defects per unit of product.
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For example when 26 defects (flaws) are found on 10 units of product, the DPU is 26/10 or 2.6 defects per unit.
Defects per Million Opportunities (DPMO) – a ratio of the number of defects (flaws) in 1 million opportunities when an item can contain more than one defect. To calculate DPMO, you need to know the total number of defect opportunities.
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For example, a form contains 15 fields of information. If 10 forms are sampled and 26 defects are found in the sample, the DPMO is:
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Parts per Million Defective (PPM) – the number of defective units in one million units. (PPM is typically used when the number of defective products produced is small so that a more accurate measure of the defective rate can be obtained than with the percent defective.)
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For example, a sample of 50 cell phones finds that 3 of them are defective. The PPM defective is then:
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Rolled Throughput Yield (RTY) (also known as the First Pass Yield) – the probability (or percentage of time) that a manufacturing or service process will complete all required steps without any failures. Reliability principles are the basis for calculating the rolled throughput yield. The reliability formula for a system in series with n process steps is:
Rs = (R1) (R2) (R3) (R4) … (Rn)
Since the reliability of a process step is the yield of that process step when quality is the performance metric, this formula then becomes:
RTY= (Y1) (Y2) (Y3) (Y4) … (Yn) where Y is the yield (proportion good) for each step
For example, a four-step process has a yield of 0.98 in step 1, 0.95 in step 2, 0.90 in step 3, and 0.80 in step 4.
RTY = (0.98)(0.95)(0.90)(0.80) = 0.67032
This means that only 67.032% of the units completed on this process will make it through all four steps without needing any rework or repair.
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