In quality management IRR stands for Internal Reject Rate, which reflects the cumulative rejection rate
of a given process chain (sum of sub-processes).
IRR is the product of individual rejection rates (fall-off-rates) of all process steps in the production
chain (including manufacturing stations and test gates as well). Practically, we can state that IRR and the Rolled Fist Pass Yield
IRR + Rolled FPY = 100%
To calculate IRR, we have to use the individual reject rates of our processes (or their counter-part: the
individual FPY of each process).
IRR = 100% - [ (100% - Reject rate 1) * (100 - Reject rate 2) * (100 - Reject rate 3) ]
IRR = 100% - [ FPY1 * FPY2 * FPY3 ]
IRR = 100% - Rolled FPY
Example: our 3 processes generate 2%, 9.09% and 6.31% reject rates, taking both scrap and rework into account.
IRR = 100% - [ (100% - 2%) * (100% - 9.09%) * (100% - 6.31%) ]
IRR = 100% - [ 98% * 90.91% * 93.69% ]
IRR = 100% - 83.47% = 16.53%
Calculation of IRR (Source: qMindset.com)
Concentrate on all process steps, not just the final test results. Based on the previous example, if a quality
manager checks only the end of line test result, he sees a much better image, instead of seeing the whole process results (6.31% reject rate vs
IRR is in close conjunction with yield metrics, such as:
- FTY (first time yield): only calculates with input and output quantities of a process step, and does not take rework into account.
- FPY (first pass yield) or TPY (throughput yield): takes rework into account.
- RTY (rolled throughput yield): the product of TPY of all processes in the production chain. It is the overall throughput yield that
takes rework and all defect scenarios into account.
Such as RTY, IRR also handles reworked pieces as rejected, so it is highly recommended to be used as input
for process improvement.