Value Stream Mapping Math: Rolled throughput Yield

Value stream analysis is an effective way to identify improvement opportunities within a product or service family’s value stream, envision a leaner future state and develop an actionable value stream improvement plan to achieve the future state. It's bread and butter stuff for the lean practitioner. Most folks are well acquainted with the value stream map’s lead time ladder. And many people are familiar with the concept of rolled throughput yield. However, based upon my humble observation of hundreds of value stream maps, there are precious few who incorporate a rolled throughput yield (RTY) line within their maps. It’s a shame. The process yield data is typically already captured in each process’ data box. From there, it doesn’t take too much effort to build out the RTY line. Know that yield represents the percentage of the process output(s) – fabricated part, assembly, analysis, transaction, reports, etc. that do not require any sort of rework or replacement, at any time. In the service and healthcare industry, this includes completeness and accuracy, the first time. Granted that the yield data contained within a current state map is often in the SWAG (scientific wild a** guess) or plain old WAG accuracy category, it still can generate a very insightful RTY line. It is painful, but not surprising to see current state RTY’s in the single digit range (i.e., =9%) …or worse. Think of that as opportunity. The RTY line captures the discrete process yield directly below the related process box and lower lead time ladder rung. As you can see in the figures, it is “boxed in.” The discrete process yield boxes are connected via a horizontal line. In between each box is recorded the cumulative or rolled throughput yield. By the very end of the RTY line, we have the full RTY for the value stream. Things can get a little bit funky when there are branches within the map. See the second figure for an example. It essentially applies a weighted average yield. In summary, make your value stream maps more useful. Add a RTY line! Related post: Value Stream Mapping Math: Lead Time Ladder Process “Branch”