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First pass yield(FPY), also known as throughput yield(TPY), is defined as the number of units coming out of a process divided by the number of units going into that process over a specified period of time.Only good units with no rework or scrap are counted as coming out of an individual process.
Also related, "first time yield" (FTY) is simply the number of good units produced divided by the number of total units going into the process. First time yield considers only what went into a process step and what went out, while FPY adds the consideration of rework.
Consider the following:
You have a process that is divided into four sub-processes: A, B, C and D. Assume that you have 100 units entering process A. To calculate first time yield (FTY) you would:
(# units leaving the process as good parts) / (# units put into the process) = FTY
The total first time yield is equal to FTYofA * FTYofB * FTYofC * FTYofD or .9000 * .8889 * .9375 * .9333 = .7000.
You can also get the total process yield for the entire process by simply dividing the number of good units produced by the number going into the start of the process. In this case, 70/100 = .70 or 70% yield.
The same example using first pass yield (FPY) would take into account rework:
(# units leaving process A as good parts with no rework) / (# units put into the process)
First pass yield is only used for an individual sub-process. Multiplying the set of processes would give you Rolling throughput yield (RTY). RTY is equal to FPYofA * FPYofB * FPYofC * FPYofD = .8500 * .8889 * .8125 * .8267 = .5075
Notice that the number of units going into each next process does not change from the original example, as that number of good units did, indeed, enter the next process. Yet the number of FPY units of each process counts only those that made it through the process as good parts that needed no rework to be good parts. The calculation of RTY, rolling throughput yield, shows how good the overall set of processes is at producing good overall output without having to rework units.