Various quality metrics serve the measurement and evaluation of manufacturing processes from quality and efficiency standpoint. Being part of an efficient production chain, quality is as important as time. Among several quality related indicators, the most useful yield metrics are the FPY (First Pass Yield) and the RTY (Roll Throughput Yield).

Both of them represent the chance of manufacturing products that meet the specification the first time, without being scrapped, reworked, or retested, however there is a slight difference between the two of them (see later).

Running a manufacturing process with high FPY or RTY means proper production performance, less waste (e.g. scrap, rework), and by this high efficiency.

The basics of yield metrics starts with FTY (First Time Yield), which is the simplest yield metric. Simplicity itself is not a problem, but FTY has a major disadvantage. It does not take rework / retest into account.

Let’s take an example of a manufacturing chain with 3 processes. 100 units enter the first process, 2 units need to be scrapped, and 8 pieces have to be reworked. As FTY does not take rework into account, the FTY of our first process will be 98%, even though we generated a lot of "waste", by reworking 8% of the entering quantity.

Based on this analogy, our overall FTY calculation will look like this:

FTY1 is (100 entered - 2 scrapped) / 100 entered = 0.9800

FTY2 is (98 entered - 5 scrapped) / 98 entered = 0.9489

FTY3 is (93 entered - 10 scrapped) / 93 entered = 0.8924

Overall (rolled) First Time Yield = FTY1 * FTY2 * FTY3

Overall FTY = 0.9800 * 0.9489 * 0.8924 = 0.8298 = 83%

In order to handle auxiliary and unproductive processes, the next metric considers rework / retest, etc. a waste. First Pass Yield (FPY) calculates with the number of passing units without rework. Based on that, the formulae is the following:

Having the same process chain, our calculation will be the following:

FPY1 = (100 entered – 2 scrapped - 8 reworked) / 100 entered = 0.9000

FPY2 = (98 entered – 5 scrapped - 10 reworked) / 98 entered = 0.8469

FPY3 = (93 entered – 10 scrapped - 5 reworked) / 93 entered = 0.8387

Overall (rolled) First Pass Yield = FPY1 * FPY2 * FPY3

Overall FPY = 0.9000 * 0.8469 * 0.8387 = 0.6392 = 64%

Although the process chain has produced 83 units that are in specification, FPY shows a much worse result compared to the calculation of FTY (64% vs 83%). The 64% value of the FPY in the example tells us, that one unit out of three products can be expected to be scrapped or reworked.

However FPY takes auxiliary processes into account, there is a bit more improved yield metric, which is the Rolled Throughput Yield (RTY). RTY represents the probability of producing good units the first time, so what is the chance that a product goes through the whole process chain defect free.

RTY not only considers rework, retest or any other issue, but improves the theory further, by taking defect distribution into account. It calculates with the number of defects. Why is it so important?

Let’s assume you have 100 units, that enter a process, and the number of passing units is still 100, after 99 pieces pass first time, and one is reworked. In such a case the First Time Yield is 100%, as the passing quantity is equal to the entering quantity. First Pass Yield reflects the one piece that has been reworked, so FPY is 99%. As seen on the chart, RTY is even better to represent quality, by calculating 3 defects, resulting an RTY of 97%. Note: in quality and Lean Six Sigma literature, RTY is often given as the product of the FPY of each process, but it is only true, if there is only 1 defect on each failed unit. In case of a part has more failure modes, it much more erodes the Rolled Throughput Yield, than the First Pass Yield.

Rolled Throughput Yield provides the rate, how capable the process is (with how much chance) to produce a defect-free unit. The following chart shows the different yield calculations for each method in the same situation. As we see, FTY calculates only with scrap, FPY takes rework into account, while RTY counts in all defects, giving the most strict result.