SPC vs. 100% Inspection - Is SPC still value-added when 100% inspection is in place?

B

brahmaiah

#11
Re: SPC vs. 100% Inspection - Is SPC still value-added when 100% inspection is in pla

Don't forget, you have special causes that can happen between sampling - and sampling may not catch it if it is intermittent. SPC is sampling - and has its limitations. So, even though your process is in control...100% inspection may be needed for certified output.

Doesn't say I like it...just a warning.

Also, you may need 100% inspection if you are using the wrong SPC - like X-bar-R for precision machining.
To say that assignable caues can escape detection in x-r chart is not correct.If such a thing is a possibility in any process you have to increase the frequency of sampling and even you can increase the sample size.100% INSPECTION IS NO SOLUTION IN A MASS PRODUCTION SITUATION.
V.J.Brahmaiah
 
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Jim Wynne

Staff member
Admin
#12
Re: SPC vs. 100% Inspection - Is SPC still value-added when 100% inspection is in pla

To say that assignable caues can escape detection in x-r chart is not correct.
It is correct. As Bob suggested, an intermittent cause can happen between samples.
If such a thing is a possibility in any process you have to increase the frequency of sampling and even you can increase the sample size.
But how is it possible to be aware of all possible process failure modes, 100% of the time?

100% INSPECTION IS NO SOLUTION IN A MASS PRODUCTION SITUATION.
Great. Tell us where the dividing line is between not-mass-production and mass production so we can know when 100% inspection is OK.
 

bobdoering

Stop X-bar/R Madness!!
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#13
Re: SPC vs. 100% Inspection - Is SPC still value-added when 100% inspection is in pla

To say that assignable cause can escape detection in x-r chart is not correct. If such a thing is a possibility in any process you have to increase the frequency of sampling and even you can increase the sample size.
Actually it is correct if the condition does not persist long enough to be detected by the sampling for the charting. If it is very infrequent, you will need to increase you sample size....to 100%

INSPECTION IS NO SOLUTION IN A MASS PRODUCTION SITUATION.
Actually, there are some very effective 100% inspection systems using vision systems, lasers, etc. These are very common in automotive where low failure rates are still not acceptable.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#14
Re: SPC vs. 100% Inspection - Is SPC still value-added when 100% inspection is in pla

Brahmaiah: perhaps this is a wording problem. in your first sentence you say you must increase your sample size and in the last you say that 100% inspection is no solution. these seem to be contradictory statements...perhaps you could clarify your meaning?
 
B

brahmaiah

#15
Re: SPC vs. 100% Inspection - Is SPC still value-added when 100% inspection is in pla

Brahmaiah: perhaps this is a wording problem. in your first sentence you say you must increase your sample size and in the last you say that 100% inspection is no solution. these seem to be contradictory statements...perhaps you could clarify your meaning?
When I said we should increase sample size, I didnot mean we should increase it to 100%.I meant we should increase the sample qty.Because 100% is not a sample.
V.J.Brahmaiah
 
B

brahmaiah

#16
Re: SPC vs. 100% Inspection - Is SPC still value-added when 100% inspection is in pla

Actually it is correct if the condition does not persist long enough to be detected by the sampling for the charting. If it is very infrequent, you will need to increase you sample size....to 100%



Actually, there are some very effective 100% inspection systems using vision systems, lasers, etc. These are very common in automotive where low failure rates are still not acceptable.
True but we are keeping them out of the perview.We are cosidering only manual inspection.
V.J.Brahmaiah
 
V

Vicman

#17
Re: SPC vs. 100% Inspection - Is SPC still value-added when 100% inspection is in pla

I insist on using SPC instead of 100% inspection. And try to keep my process under control with a cpk above 1.66.
If we remember history, sampling plans were first use by military suppliers and because of high rejections, finally were replaced by SPC techniques.

Something that can help here is to use other tools besides SPC like Poka Yoke.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#18
Re: SPC vs. 100% Inspection - Is SPC still value-added when 100% inspection is in pla

I insist on using SPC instead of 100% inspection. And try to keep my process under control with a cpk above 1.66.
If and only if all of the failure modes you have identified in your FMEA will persist when they occur, then the sampling rate used for SPC will allow you to not need to 100% Inspect.

If we remember history, sampling plans were first use by military suppliers and because of high rejections, finally were replaced by SPC techniques.
SPC is an improvement over sampling by itself - but is still based on sampling and exhibits some of its limitations. In fact, some of the variation in SPC charts can be attributed to sampling error - not entirely the process. SPC is not foolproof and should not be expected to detect 100% of the defects. If 100% good product is the customer requirement (critical characteristic) You may need 100% inspection to verify the product in addition to SPC to control the process to avoid rejects.

Something that can help here is to use other tools besides SPC like Poka Yoke.
If a poka yoke can be developed for the condition, it can be an effective approach in that it verifies 100%
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#19
Re: SPC vs. 100% Inspection - Is SPC still value-added when 100% inspection is in pla

True but we are keeping them out of the perview.We are cosidering only manual inspection.
V.J.Brahmaiah
we are? I ddint' see that in the OP's post or anyone else's response. did I miss something? but beyond that why woudl we keep them out of the perview as you say?
 
B

brahmaiah

#20
Re: SPC vs. 100% Inspection - Is SPC still value-added when 100% inspection is in pla

we are? I ddint' see that in the OP's post or anyone else's response. did I miss something? but beyond that why woudl we keep them out of the perview as you say?
100% Inspection is justifiable only in aerospace&medical Products. .If you depend on it in automotive and consumer industry.You cannot stand in the global competition.The success of japanese is there to see for all of us.
V.J.Brahmaiah
 
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