Random Sampling at Receiving Inspection: A Practical Implementation needed

Jim Wynne

Staff member
Admin
#11
Hi Jim,

Our inspection process is working well. It is mostly AQL based and we have a QC team that is skilled to handle all required inspections. The issue is strictly on how to avoid 'judgement bias' in the selection of the sample to be inspected.

I was surprised to find elaborate ISO standards on the issue but almost no relevant discussions in forums on this topic.

Paul
This whole subject is one that isn't discussed much. You can find a lot of discussion here on sampling plans in general, and we have some very knowledgeable people, but the fact is that most people just go through the motions when it comes down to actual lot sampling. As I suggested earlier, it's often just not practical to do orthodox random sampling.

This is one of those areas where theoretical statistics and actual business practices collide, and the results of the collision are as predictable as the aftermath of a collision between a Sherman tank and a Smart Car. The business side will always win.

I think that most CB auditors, at least the ones I've dealt with, are aware of this and don't make a big deal of it. Sometimes the best we can do is to make a conscientious effort to select a representative sample. The other way to avoid these issues is to push the responsibility back on suppliers, where it belongs. If suppliers have good control over their processes, the need for sampling inspection can be greatly reduced and even eliminated in some cases.
 
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harry

Super Moderator
#12
So the question remains: can forum members comment if they have implemented formal randomization methods (e.g. based on ISO 24153) in their receiving inspection?
I think you have to have an understanding of the product you are going to sample, its behavior and the manufacturing process that it underwent. I had experience with bulk material sampling and for liquids for example (blended oil), samples are usually collected at 2-3 spots along the tank and at least at 3 different depths.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#13
first: observations do not typically require an answer or an action to improve.

that said - let's move on to your specific request: there are two primary reasons why randomization schemes are not discussed much
  • the schemes are so specialized as to how parts or material is 'packaged' that there are a lot of combinations
  • many organizations simply don't use random sampling.
I'm one of those that beleives that if you aren't going to do random acceptance sampling it's probably not very effective.
In my organization we have enough volume of each our part numbers that we can (although the initial setup is tedious - I used an intern!) establish a set pattern for each product type. (our sample sizes don't change much as we don't use the ANSI tables that are tied to lot size; we use straightforward sample size calculations based on our RQL where lot size is not part of the equation) We simply map the packaging breaking it into appropriate zones, and randomize the zones in a random order list.

for example: I have a pallet that has 200 boxes in it. each box position becomes a numbered grid. the 200 positions are then sorted in a random order. lets say my sample size is 22. I will go to the sorted order and sample the top 22 positions (boxes). If each box has 10 parts in it and those 10 parts are packaged in a grid, I will create a random order of the 10 part positions for each box. (unique for each box) the first in-box position for each box is the part that is selected for that box. If for some reason, I only get a half pallet, I skip the pallet positions that are not available. if I get two pallets, I take the first 44 positions in the list usign the first 22 for one pallet and the second 22 for the secodn pallet. I always use these pallet positions.

yes - it takes time to set up, but once done the maintenance is easy and minimal. The inspector has a random selection list for each part, just as s/he has a different set up criteria and specifications for each part.
 
#14
Hi Bev,

This is very helpful!

You appear to have resolved the issue that I am faced with. I would be very interested to learn more about how you have approached the various packing methods. Those methods that use some kind of stacking of boxes and trays appear to be most straightforward. How do you handle bulk materials?

Do you use random tables that are specific to a package?

I like your usage of an intern for such project, I may do the same.

Paul
 
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