Sampling Multiple Times from One Master Lot

R

Rogersr

#1
Hello,
As a means of reducing our on-hand inventory, a vendor of ours keeps a just-in-time inventory of various items in their US warehouse after it is built overseas. While this is beneficial for us in terms of space, it means that currently we might receive 20 incoming lots of 50 pieces rather than just receiving 1000 pieces at one time. Overall this increases our QC Inspection time due to the non-linear relationship betweeen sample:lot size and the paperwork that must be completed each time. Since we know what the overall lot size is and since the product is built to our specification, I am wondering if there is any reason why we couldn't perform our sampling based on the overall lot size when the first shipment (or two or three) arrive, then disregard sampling on all subsequent mini-lots from the same master lot until a new lot is built. I know this would require careful bookkeeping in order to ensure that new lots are flagged, but I suspect it would save time in the long run. To be honest, this idea was suggested to me and I am quite hesitant to try it, but don't necessarily know why.
In case it helps, we are sampling using the ANSI Z1-4 plan and our inspections consist of attributes and variables (wire length, wire continuity, etc.) The vendor's quality system is ISO 9001.
Thanks in advance,

Bob
 
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Jim Wynne

Staff member
Admin
#2
Hello,
As a means of reducing our on-hand inventory, a vendor of ours keeps a just-in-time inventory of various items in their US warehouse after it is built overseas. While this is beneficial for us in terms of space, it means that currently we might receive 20 incoming lots of 50 pieces rather than just receiving 1000 pieces at one time. Overall this increases our QC Inspection time due to the non-linear relationship betweeen sample:lot size and the paperwork that must be completed each time. Since we know what the overall lot size is and since the product is built to our specification, I am wondering if there is any reason why we couldn't perform our sampling based on the overall lot size when the first shipment (or two or three) arrive, then disregard sampling on all subsequent mini-lots from the same master lot until a new lot is built. I know this would require careful bookkeeping in order to ensure that new lots are flagged, but I suspect it would save time in the long run. To be honest, this idea was suggested to me and I am quite hesitant to try it, but don't necessarily know why.
In case it helps, we are sampling using the ANSI Z1-4 plan and our inspections consist of attributes and variables (wire length, wire continuity, etc.) The vendor's quality system is ISO 9001.
Thanks in advance,

Bob
Welcome to the Cove :D

If you can assure that your sample is truly random (all parts in the "master" lot have an equal chance of being selected) and you know that the integrity of the inspected lot will be maintained by the supplier, then there's no reason not to do what you're suggesting.
 

Eric ng

Involved In Discussions
#3
:)
Hello,
As a means of reducing our on-hand inventory, a vendor of ours keeps a just-in-time inventory of various items in their US warehouse after it is built overseas. While this is beneficial for us in terms of space, it means that currently we might receive 20 incoming lots of 50 pieces rather than just receiving 1000 pieces at one time. Overall this increases our QC Inspection time due to the non-linear relationship betweeen sample:lot size and the paperwork that must be completed each time. Since we know what the overall lot size is and since the product is built to our specification, I am wondering if there is any reason why we couldn't perform our sampling based on the overall lot size when the first shipment (or two or three) arrive, then disregard sampling on all subsequent mini-lots from the same master lot until a new lot is built. I know this would require careful bookkeeping in order to ensure that new lots are flagged, but I suspect it would save time in the long run. To be honest, this idea was suggested to me and I am quite hesitant to try it, but don't necessarily know why.
In case it helps, we are sampling using the ANSI Z1-4 plan and our inspections consist of attributes and variables (wire length, wire continuity, etc.) The vendor's quality system is ISO 9001.
Thanks in advance,

Bob
Hi Bob,

You still can use the same sampling plan, except the sampling method has to change as one big lot of 1000 is now divided into 20 small lots of 50 each. As long as you follow random sampling principle as Jim's has mentioned, that is, the required sample size is randomly selected from the 20 sub-lots, there shouldn't be any problem.

Eric
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#4
getting a random sample is the difficult part in all likelihood. the boxes are probably packaged FIFO and so you'd have to open several boxes and keep counts straight so you ship the correct quantity, etc.

the BIG question is: why do you need to do IQA inspection at all?????
 
R

Rogersr

#5
Thanks to all who have replied. I agree that the big issue will be to ensure that the samples are random.
 
R

Rogersr

#7
Bev,
Good question- I apologize for not replying more quickly.
At this point we need IQA because our company relies heavily on overseas vendors that have not demonstrated a reasonably high quality level. Unfortunately, this is complicated by our company's early history of not adequately defining what we needed or expected, so it is not always a matter of their shortcomings, but also a matter of our inadequacies.
I've been at the company about 5 months and have been able to make some good changes, but getting people to understand what we lose by staying with substandard vendors is a tough sell thus far. Hopefully in time...

Bob
 

Ajit Basrur

Staff member
Admin
#8
Hi Rogersr,

The idea is very good and you have received very good responses so far.

I would just like to add 2 more points to ensure that you are in compliance at all times -

1. Ensure that the proper storage conditions (if any) are maintained at supplier end.

2. In your Supplier Audit Checklist, ensure that you audit the storage condtion, inventory of those materials, proper status labelling maintained etc.
 
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