Normalized vs. Rolled Throughput Yield - First Pass Yield metrics for our processes

Everyone,

I am looking for some assistance on this one, Rolled Throughput Yield vs. Normalized Yield.

We have a customer that insists on First Pass Yield metrics for our respective meter assembly processes which consist of a 12 Station Process. Out of these individual stations we compile various defects some station performance related & some meter form, fit, function related.

Issue is they are all lumped together in the process tracking database. We will have to filter data for the meter specific defects - not too big a challenge. The customer only needs to be concerned with the meter specific information, as these are issues that will be seen potentially by them.

I have gone a little divergent to frame the current situation.

Customer visits for an audit observes series of meters failing, asks what is the First Pass Yield for the station - stats are pulled and it appears that is is performing below goal. However, the optical camera needed adjusting after this values meet and exceed target.

Customer asks for the Rolled Throughput Yield for the Entire Process:

Rolled Throughput Yield (RTY) = Station 1 * Station 2 * .... Station *12

We report the RTY yield - Let's say for discussion it was a theoretical number such as 87% - they say Automation Process at Risk in meeting Customers demands of 95%.

Is it just me that sees how difficult this is to achieve - .995 ^ 12 = .9416 not quite good enough. If all the stations performed at a FPY = 99.5%, we would still miss the mark.

This is why I like the concept of normalized yield:

Normalized Yield = nth Root (Rolled Throughput Yield) = .87 ^ .0833 = .988

They would be happy with 98.8%, but how do I explain the Theory behind this. This is the expected FPY for stations in the process, which I think they really want vs. the chance of a single meter getting through the process defect free.

Rework is a fact of life for a new start up process until the learning curve gets firmly established.

HELP WITH THE THEORY BEHIND THIS, SO I CAN EXPLAIN NORMALIZED YIELD EFFECTIVELY TO OUR CUSTOMERS AND SELL THEM ON IT!!!!!



Any feedback on this will be greatly appreciated!!!!

Thanks.
Eric Quesnel
QC Manager - Itron

Re: Normalized vs. Rolled Throughput Yield - First Pass Yield metrics for our process

See this article in Quality Digest regarding RTY for a clear discussion on why RTY is used.

Normalized yield is just the typical yield you would expect at one single operation, if the defect are spread evenly across each process step. It can be very misleading and does not represent the total process.

Your customer is correct in focusing on RTY.

Re: Normalized vs. Rolled Throughput Yield - First Pass Yield metrics for our process

FPY is actually for individual process, and therefore customer or the boss would always like to look at the RTY. The reason is RTY is the actually output but FPY only show the individual process lose.

1000 material go to the 10 process, each of the process provide 99% FYP, and finally we can find that only 904 output and the finish good area. That's what customer interested.

Besides, different process suppose to has different FPY, not necessary to have same target but set it to fulfill the RTY target given by top management or customer.

Re: Normalized vs. Rolled Throughput Yield - First Pass Yield metrics for our process

I would agree with this fact if indeed the parts (meters) were truly scrap parts, but the recovery rate is about 90% of those that do not make FPY.

I understand we are adding labor that at this point is Non-Value Add, but with this being such a new product the rate they are expecting is very difficult to achieve.

What do you know to be the industry standard for RTY in your industry??

Thanks for your reponse!!

Re: Normalized vs. Rolled Throughput Yield - First Pass Yield metrics for our process

equesnel said:

I would agree with this fact if indeed the parts (meters) were truly scrap parts, but the recovery rate is about 90% of those that do not make FPY.

I understand we are adding labor that at this point is Non-Value Add, but with this being such a new product the rate they are expecting is very difficult to achieve.

What do you know to be the industry standard for RTY in your industry??

Thanks for your reponse!!

Click to expand...

Different industrial has different standard. Six sigma qualified manufacturing company able to achieve 99% RTY for certain product.

As i know the customer who is looking for this kind of data normally is lean manufacturing company.They always looking for "fast delivery" and therefore they very care about your RTY. RTY directly telling them the quantity you can delivery to them in one cycle time.

Ops, too far from the topic...lol. Let's go back to the topic

Adding labour generally is non value added but not necessary. Most of the lean guy will tell you that you shouldnt add this and that, this and that is non value added. That's not true. Why? How about i add a new line? Of course i need to add labours. Lean is to reduce the waste and improve the effectiveness, so make a VSM show the result. Adding labour to reduce the WIP (waiting times) will increase your value added percentage.

"Dont add the labour" is establisshed under a prefect process balancing. If the initial man power arrangement is not good, then add labour is one of the action to make better process balancing (reduce the WIP). Again, use VSM to calculate it before you add labour

By the way, lean manufacturing company is actually just "try their best" to helps their supplier to make them into lean but not necessary too. The only thing they need to confirm is supplier able to give them ontime delivery, else they will facing big problem (This is the disadvantage of zero stock)

Re: Normalized vs. Rolled Throughput Yield - First Pass Yield metrics for our process

You really need to understand what your customer is interested in.
The two different yield calcualtions have different purposes.

I always insist on RTY for my suppliers as I want to understand Quality and ability to Deliver and Cost to deliver...of course my goals for RTY improvement are that my suppliers are continually improving I rarely have a hard goal line. and your custoemr will also probably be OK with an active plan to improve...talk to them, understand their need, then we can talk about 'selling'...

Re: Normalized vs. Rolled Throughput Yield - First Pass Yield metrics for our process

Bev. - Thanks for this comment!! What we have been able to sucessfully do is get them interested in the Continuous Improvement aspect of the business. We have been able to effectively show postive process capability trending, even if it is not exactly to the levels they are looking for yet.

Thanks for your comments.

Re: Normalized vs. Rolled Throughput Yield - First Pass Yield metrics for our process

i work in a electronic assembly company we want do FPY but in most cases we can repair it right on the line so my question is if 5 units fail and get repaired before next operation explain how i would look at FPY in this situation. PLease help

Re: Normalized vs. Rolled Throughput Yield - First Pass Yield metrics for our process

I'm not sure why this poses a problem for you?
If 10 units come to station A and 5 are found to be defective then the FPY is 50%. Where the units are reworked doesn't affect the philosophy or the math - the fact that they required rework is what is important.

Re: Normalized vs. Rolled Throughput Yield - First Pass Yield metrics for our process

If this is a Printed Circuit Board for example I would track Defects Per Million Opportunities (DMPO) instead of First Pass Yield (FPY).

However, to answer your question - if the part has to have rework completed it is a first pass failure. Even if the work is completed real-time, it still did not pass the first time at the specific testpoint.

I hope this answers your question - there may be other perspectives on this one, but I think this is the only true answer in my opinion.