When working on a Six Sigma project, accurate methods to measure process performance metrics are critical for understanding the current state of a process and the value of changes made. Four of the most common measurements are Defects Per Unit (DPU), Defects per Million Opportunities (DPMO), Parts per Million Defective (PPM), and the Rolled Throughput Yield (RTY).
The following provides how each is used. However, it’s important to first understand the difference between two terms commonly used in connection with these performance measurement tools. The first is “defect.” The second is “defective.”
Another way to look at this is opportunity vs. units. A unit is the final product delivered to a customer. It can contain many defects and be found to be defective. Opportunities represent everything that goes into making a unit – materials, labor, delivery, etc. Each of these opportunities has the potential of having a defect.
DPU measures the average number of defects per every product unit. It’s found by dividing the total number of defects found by the number of units.
For example, if 30 units are produced and a total of 60 defects have been found, the DPU equals 2.
This represents a ratio of the number of defects in one million opportunities. In other words, how many times did you have a flaw or mistake (defect) for every opportunity there was to have a flaw or mistake.
For example, consider a form that contains 15 fields of information. If 10 forms are sampled and 26 defects are found in the sample, the DPMO is:
It’s also possible to translate DPMO to a Six Sigma level. The goal is to reach 3.4 defects per 1 million opportunities.
The PPM represents the number of defective units per 1 million units. Again, using the car scenario, the PPM would include the total number of defective cars – cars determined to be too flawed to be sold – per every 1 million cars manufactured.
PPM is arrived at by simply taking the number of defective units in a same size, dividing that number by the total sample size, and multiplying by 1 million.
For example, a sample of 50 cards found that three are defective. The PPM defective is then:
RTY (also known as the First Pass Yield) measures the probability (or percentage of time) that a manufacturing or service process will produce a defect-free unit. This requires mapping out a process to determine how many steps it involves.
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.
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.
Once an organization understands the different performance measurement tools and how to use them, the important next step is to determine how to calculate baseline sigma and determine what other metrics to measure.
Unlike DPU, which gives you a better understanding of how many units to expect to leave the process with errors, DPO gives you an understanding of the true failure chance for a defect to occur. In the example above, the DPU, or
unit, is 0.06, or a 6% chance of a unit having a failure.
Example of calculating DPO Each custom stationary order could have four defects – incorrect, typo, damaged, or incomplete. Therefore, each order has four opportunities. Fifty orders are randomly selected and inspected and the following defects are found.
Six of the orders have problems, and there are a total of 7 defects out of the 200 opportunities (50 units * 4 opportunities / unit); therefore DPO = 7/200 = 0.035.
Using DPU, DPO, and DPMO to assess defects
DPU, DPO, and DPMO are metrics that express how your product or process is performing, based on the number of defects. Choosing the appropriate quality metric helps you assess performance against customer expectations. You can also develop project baselines and improvement goals, as well as communicate the level of conformance to your customers.
How do you calculate parts per million defects?
How do you calculate DPU and DPO?
There are a total of 7 defects out of the 200 opportunities. Therefore, DPO = 0.035 and DPMO = 0.035 * 1000000 = 35,000. If your process remains at this defect rate over the time it takes to produce 1,000,000 orders, it will generate 35,000 defects.
What is DPU quality?
What is DPU formula?
It’s found by dividing the total number of defects found by the number of units. For example, if 30 units are produced and a total of 60 defects have been found, the DPU equals 2.
What is my DPU?