100% Inspection - 85% Effective

M

Mark Paul

#1
Who came up with the concept that 100% inspection is only 80-85% accurate or effective?

If I am doing a visual inspection sorting like parts for paint, no paint, and wrong color paint, what is the expected percentage that all will be caught through visual inspection in 100 or 1000 parts? :confused:

I am looking for where to look it up moreso than an mathmatecal answer but I'll take that if someone has it.
 
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Jim Wynne

Staff member
Admin
#2
Who came up with the concept that 100% inspection is only 80-85% accurate or effective?

If I am doing a visual inspection sorting like parts for paint, no paint, and wrong color paint, what is the expected percentage that all will be caught through visual inspection in 100 or 1000 parts? :confused:

I am looking for where to look it up moreso than an mathmatecal answer but I'll take that if someone has it.
The bromide in question has no empirical basis, so there's really nothing to look up. Depending on the circumstances, 100% inspection might be 100% effective or might be 10% effective.

The efficacy of any inspection process depends on:

  • Objective criteria
  • Well-trained and qualified inspectors
  • The number of things to be inspected
  • The number of attributes in question
As well as other things, such as lighting, operator fatigue and time constraints. The definition of "effective" also plays a part; one might consider finding x% (some number less than 100%) of the defectives in a given lot to be an acceptable outcome.
 
T

tsmith7858

#3
I took a stat/probability course years ago where we did an experiment to show that 100% inspection was not 100% effective. We all read a paragraph and had to pick out the defects (three different letters were deemed defects). We were timed to duplicate stress and our training was a brief "how to" before the experiment.

In a class full of Quality Inspectors, Supervisors and Managers we managed an 89%.
 
S

somerqc

#4
I have been involved in classes like that as well. We were within the same range (I have been in classes as high as 95% and as low as 85%).

I have also worked where top management wanted 100% inspection. We were constantly dealing with parts that got through inspection.

The other issue with 100% is that that if you have a "gradual" change in a product, it is very difficult to determine the exact point where the change took place (i.e. some quantity of rejects go by until the change is significant enough for the inspector to notice.). This is due to human nature - not a lack of training or competence.

To add to that, the cost of 100% is extreme compared to an effective sampling plan with almost the same results.

Just my :2cents: - but you will not find a statistical study saying exactly how ineffective 100% inspection is (well, I haven't found one yet anyway).
 
A

andygr

#5
It is the quality version of an urban myth.
The effectivity of inspection is based on differances you are trying to discriminate. It realy depends on who and what is being inspected.
sorting elephants from mice is 100% effective except on fri afternoon close to quiting time.:biglaugh:

:2cents:
 

Jim Wynne

Staff member
Admin
#6
I took a stat/probability course years ago where we did an experiment to show that 100% inspection was not 100% effective. We all read a paragraph and had to pick out the defects (three different letters were deemed defects). We were timed to duplicate stress and our training was a brief "how to" before the experiment.

In a class full of Quality Inspectors, Supervisors and Managers we managed an 89%.
That's a fairly common demonstration, and I think it's useful to make people aware that inspection might not be as easy as it looks. Nonetheless, the results shouldn't be considered definitive in any way. Like I said, the efficacy of any sort of inspection is dependent on a lot of different variables, and each case should be evaluated independently.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#7
To add to that, the cost of 100% is extreme compared to an effective sampling plan with almost the same results.
Generally, an effective sampling plan verifies the product, and at a particular failure level will require 100% inspection as reaction plan. The lower the frequency of a failure, the closer the effective sampling plan is to 100% inspection, anyway.

The real issue is more closely associate with Gage R&R. 100% visual inspection of missing components will not have a failure rate similar to a visual inspection with levels of acceptable variation, such as porosity, rust, scratches, etc. Also the geometry is important. 100% inspection all around a thread for nicks is different than looking for fisheye in paint on a flat panel. On top of that, the obviousness of the condition, such as missing threads versus missing nickel plating on a steel part, offer much different visual inspection failure rates.

The point is not whether 85% is a statistical fact, but rather the point that 100% inspection is offers a significant risk of not being 100% effective. In fact, 200% inspection also offers the risk of not being 100% effective - even though it is often the second option used when 100% inspection lets you down.
 

Jim Wynne

Staff member
Admin
#8
The point is not whether 85% is a statistical fact, but rather the point that 100% inspection is offers a significant risk of not being 100% effective.
It might present significant risk. The problem with this sort of thing--a type of quality Wikipedia reference-linkmeme--is that it's so often mindlessly accepted as Received Truth, and instances where 100% inspection is a perfectly acceptable choice are avoided for fear being considered a wild-eyed heretic.
 
T

tsmith7858

#9
sorting elephants from mice is 100% effective except on fri afternoon close to quiting time.

:2cents:
I have seen enough cartoons to know you don't need inspection for this because the elphants would separate themselves when they saw the mice...:lol:
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#10
It might present significant risk. The problem with this sort of thing--a type of quality Wikipedia reference-linkmeme--is that it's so often mindlessly accepted as Received Truth, and instances where 100% inspection is a perfectly acceptable choice are avoided for fear being considered a wild-eyed heretic.
Over all possible situations, it poses a significant risk. For any particular situation, its risk level is its own...which may or may not be significant. If you have to sort 5 parts for a missing 4" hole on an outside surface visually, you have a pretty good chance of being successful for that population.
 
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