DPPM - 'Standard' minimum quantity to measure DPPM?

B

Brookway

Hi There,

One of our customers has set their acceptable DPPM level at 5K, for all their suppliers.... across the board.

The problem is we send them about 20 different p/n's totaling 130 pcs per month. The parts are extremely high tolerance machinings and inevitably one (or more) will be found out of tolerance (by .0001") in a month’s time.

I've been attempting to persuade their Global Quality Director that this DPPM requirement does not fit the highly complex low volume product type we manufacture for them. Not going to far with this.

Is there a standard minimum qty that must be measured before DPPM can be accurately calculated? Something I can throw back that might assist me in my effort?

Thanks - Brookway
 
D

Darius

Re: Is there a standard min. qty. to measure DPPM?

A difficult ones.

There is no minimum as far as I know :frust:but....

The chi square tables to determine "normality" (Gausian distribution) goes as far as 60 (more or less), so it could be a good point to start (60 minimum)

The traditional control charting take in account 20 samples of 5 data points, it means 100 data points to determine SPC control limits.

Don Wheeler said that it's better to obtain the % of defective by counting insteed of using fourmulae and statistic indicators, b'cause the asumptions on the used statistics. As you said it spected to be a % defective more than a DPPM if you find 1 or more in 130 samples

As I put in another post about cp/cpk, the index has it's uncertainly so an index cpk=1.33 could be between 0.7 and 1.9 (to say something without any math) depending on the sample size. So a % defective it self has it's own uncertainly.:nopity:
 

Statistical Steven

Statistician
Leader
Super Moderator
Re: Is there a standard minimum quantity to measure DPPM?

Hi There,

One of our customers has set their acceptable DPPM level at 5K, for all their suppliers.... across the board.

The problem is we send them about 20 different p/n's totaling 130 pcs per month. The parts are extremely high tolerance machinings and inevitably one (or more) will be found out of tolerance (by .0001") in a month’s time.

I've been attempting to persuade their Global Quality Director that this DPPM requirement does not fit the highly complex low volume product type we manufacture for them. Not going to far with this.

Is there a standard minimum qty that must be measured before DPPM can be accurately calculated? Something I can throw back that might assist me in my effort?

Thanks - Brookway


So, if I understand correctly, you ship them about 1560 parts per year. Since they have a 5K DPPM requirement, it would allow for approximately 8 defective parts per year. One defective in 130 means a rate of 7700 DPPM. There is no minimum quantity to "estimate" DPPM. The better question is not the estimate, but the variability in the estimate. Here is a table that shows regardless of aggregation by month or year gives the same estimate, but you would feel more confident with a years data over a months data

Defects/Month Monthly Estimate (Based on 130 units per month)
0 0
1 7692
2 15385
3 23077

Defects/Year Yearly Estimate (Based on 1500 units per year)
12 8000
18 12000
24 16000
30 20000
36 24000
 

Jim Wynne

Leader
Admin
Re: Is there a standard minimum quantity to measure DPPM?

In addition to Steven's good advice, the dilemma you're facing is that PPM makes no sense as a performance measure when small numbers are involved. Good luck trying to get your customer to understand this. If we follow the PPM concept logically, we have to accept that the results from a very small sample will hold true after a million parts have been produced, and without knowing a lot more about the process, there's just no good reason to make that leap.
 
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B

Brookway

Re: Is there a standard minimum quantity to measure DPPM?

Thank you for your replies. Yes, it will be a battle changing the minds of others when the outcome to them is more work. I had a similiar challenge with a different customer where we started counting features instead of actual parts, this worked out ok. I'll put together a speadsheet that calcualtes this and see if I can sell it.

We'll keep pluggin away, thanks again.
 

Statistical Steven

Statistician
Leader
Super Moderator
Re: Is there a standard minimum quantity to measure DPPM?

Thank you for your replies. Yes, it will be a battle changing the minds of others when the outcome to them is more work. I had a similiar challenge with a different customer where we started counting features instead of actual parts, this worked out ok. I'll put together a speadsheet that calcualtes this and see if I can sell it.

We'll keep pluggin away, thanks again.

As a joke, you could tell them if you improve your process such that you can acheive better than 5K DPPM, you would be happy to ship them some defects to meet the target. I would highly recommend you tell them that due to the small sample size, any estimate of DPPM would be unreliable. Just to add to my comments from before, if 5K DPPM allows for approximately 8 defects per year of production, that if you reach 7 defects you would need to stop shipping because another defect would but the process out of their acceptable region. Another point to be made, DPPM is an estimate for a stable process. You did not say if all the defects came from a single p/n or randomly from all 20 p/n you ship. Are the processes similar enough to pool for DPPM calculations? Just some thoughts.
 

Jim Wynne

Leader
Admin
Re: Is there a standard minimum quantity to measure DPPM?

As a joke, you could tell them if you improve your process such that you can acheive better than 5K DPPM, you would be happy to ship them some defects to meet the target.

Excellent. :agree1: There was a recent discussion here regarding PPM that further demonstrates that many customers don't understand what they ask for. Someone had a customer who was asking for CA in a situation where the supplier's PPM level was <1.5 and and there were millions of parts involved. Although it wasn't stated by the person raising the question, my guess is that the customer's required PPM level was something considerably greater than 1.5. Regardless of the PPM level, the customer can always reserve the right to debit for NC product, but my guess is that the most economical and sensible course of action would be to throw out the one-in-a-million and get back to work.

If a customer has a requirement for x PPM, and the supplier's performance is some number less than x, and at the same time has a CA system that requires action for any defective received, it's clear that the customer doesn't understand what's being asked for.
 
B

Brookway

Re: Is there a standard minimum quantity to measure DPPM?

Yes, they are from different parts here or there. We have had a couple months with no defects, then a bad month with (4). The parts are complicated aluminum castings that are machined, chromated, helicoils installed, then masked and painted.
 
J

Jim Shelor

Re: Is there a standard minimum quantity to measure DPPM?

Hi There,

One of our customers has set their acceptable DPPM level at 5K, for all their suppliers.... across the board.

The problem is we send them about 20 different p/n's totaling 130 pcs per month. The parts are extremely high tolerance machinings and inevitably one (or more) will be found out of tolerance (by .0001") in a month’s time.

I've been attempting to persuade their Global Quality Director that this DPPM requirement does not fit the highly complex low volume product type we manufacture for them. Not going to far with this.

Is there a standard minimum qty that must be measured before DPPM can be accurately calculated? Something I can throw back that might assist me in my effort?

Thanks - Brookway
Brookway,

If I understand your issue, you have 20 different parts, you make a total of 130 parts in a month, accordingly, you only make 6 – 7 of each part per month.

First of all, I question the validity of adding all the parts together for your DPM calculation unless the parts are identical or nearly identical.

Applying a thumbrule very loosely, to accurately determine a failure rate the sample should be large enough such that 5 or more defects would be detected in the sample.

For your case, if using all the parts as a single sample is valid, 5000 DPM equates to 5 failures in 1000 parts (8 months of production).

If the parts are different enough that combining them into single sampling group is not valid, your required number gets even worse.

My opinion is the only way to satisfy your customer’s requirement is to perform 100% inspection and rework/scrap any failed parts and replace them with good parts.

I would not go to the customer with a story that effectively says “I cannot prove I can meet your requirements so I request you change your requirements to ones I can meet”. If I was your customer, I would start the search for somebody else that can make the parts.

Sincere regards,

Jim Shelor
 

Stijloor

Leader
Super Moderator
Re: Is there a standard minimum quantity to measure DPPM?

Hi There,

One of our customers has set their acceptable DPPM level at 5K, for all their suppliers.... across the board.

The problem is we send them about 20 different p/n's totaling 130 pcs per month. The parts are extremely high tolerance machinings and inevitably one (or more) will be found out of tolerance (by .0001") in a month’s time.

I've been attempting to persuade their Global Quality Director that this DPPM requirement does not fit the highly complex low volume product type we manufacture for them. Not going to far with this.

Is there a standard minimum qty that must be measured before DPPM can be accurately calculated? Something I can throw back that might assist me in my effort?

Thanks - Brookway

Hello Brookway,

Your situation is a classic example of customers imposing unrealistic requirements that they have not (even statistically) evaluated themselves.

There are examples here in The Cove of suppliers with exemplary quality and delivery performance that were required to do a full blast root cause analysis on a one-time nonconformity where the root cause was very obvious.

I believe that it's OK to (professionally) push back a little. If the suppliers don't, that wil set a precedent and the situation perpetuates itself.

It's a matter of customer education. And the time to do this is during review of customer requirements (see 7.2.2). Amazing what this, and an open dialogue with the customer can do. You may not always "win", but it's always worth a try.

Stijloor.
 
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