Rolled Throughput Yield, or RTY, is the probability that a unit passes every step in a process first time, with no rework anywhere. It is calculated by multiplying the FPY of each step together. RTY = FPY₁ × FPY₂ × FPY₃ × ... × FPYₙ. Because every factor is below 1.0, RTY is always lower than the lowest individual step FPY and usually a lot lower than people expect.
A harness line with four steps, each at a healthy-looking 97 percent, does not deliver 97 percent right first time. It delivers 0.97 × 0.97 × 0.97 × 0.97 = 0.885, or 88.5 percent. Nearly 12 out of 100 harnesses got touched somewhere. Add more steps and the figure drops further, which is why long processes with "good" individual yields can still have crowded repair areas.
RTY is the honest measure of a whole process. If someone asks what proportion of harnesses go from wire reel to packing with nobody intervening, RTY is the answer.
Standard Procedure
- 1List every step in the process in order.
- 2Calculate the FPY for each step over the same time period.
- 3Multiply the FPYs together, one at a time, writing down each running product.
- 4State the RTY as a percentage and as "about X out of 1,000 units untouched".
Lesson Summary
- ✓RTY multiplies step yields; four steps at 97 percent give 88.5 percent, not 97 percent.
- ✓Use the same time period for every step's FPY so the product is meaningful.