Do you recognize the following? A quality issue arises and the standard response is to increase the inspection. But how effective is 100% inspection. Often these inspections are done by the quality control departments through trained quality inspectors. What can go wrong, right? Well let’s do a test.

Preparation

Write down of project the following sentence:

THE NECESSITY OF TRAINING FARM HANDS FOR FIRST-CLASS FARMS IN THE FATHERLY HANDLING OF FARM LIVESTOCK IS FOREMOST IN THE MINDS OF FARM OWNERS.

SINCE THE FOREFATHERS OF THE FARM OWNERS TRAINED THE FARM HANDS FOR FIRST-CLASS FARMS IN THE FATHERLY HANDLING OF FARM LIVESTOCK THE FARM OWNERS FEEL THEY SHOULD CARRY ON WITH THE FAMILY TRADITION OF TRAINING FARM HANDS…

  Rules and game play

Let the participants read the sentence and let them count the number of ‘F’s in the following sentence. The participants should only read the sentence once! Give them about 45 seconds.

Collect the answers.

Did somebody count 27 F’s. Then that person is right! Try is yourself! But trust me if I say that many people will have it wrong.

Lessons learned

100% inspection does not mean that 100% of the bad parts are removed.  All the participants know English and can therefore be categorized as qualified inspector and yet they have made mistakes!

It is easy for our brains to get conditioned to overlook the small things when we are performing routine work .

In this specific case we have three factors that influence our answer. 

The first factor is that the letter “f” can both have a hard and a soft sound.  In “farm” we have a hard f but is of we have a soft f. the soft f is pronounced more like a v. The hard f sound is made by touching the upper teeth to the lower lip and then breathing out. The soft f sound is made in a similar way but exactly the same way except rather than breathing out you activate your voice.

When we ask the question count the number of ‘f’s. We hear the hard f sound.  Therefore we could overlook the F’s which have a soft f sound.

The second factor is that when we read we do not focus on each word equally. Our brain jumps over simple words such as “of”, “in”, “on”, “at” and so on. The brain gives more importance to the verbs and nouns and connects the words routinely together to create sentences that makes sense.  This routine makes that we overlook certain words.

The third factor is that for our brain it doesn't matter in what order the letters in a word are, the only important thing is that the first and last letter are at the right place. The rest can be in any order and we can still read it without problem.

OUR BARIN  DEOS NOT RAED EERVY LTTEER BY ISTLEF BUT THE WROD AS A WHOLE

An “F” which is in the middle of a word will get less focus then a F which is at the beginning or end of a word.

This explains also why non-native speakers (for which English is not a routine) are often better in this exercise. They have to focus on all the words and letters within that word. And they are not mislead by hard and soft f sounds.  They really look at the pattern of letters.

The main message is: Even a trained operator will overlook defects when looking routinely to parts.

100% visual inspection does not exist

In general, they say that 100% visual inspection results in only 80% reliability. Meaning that from 100 defects we will only detect 80%. This number is of course a genal number and depends on many factors.  But it will never be 100%.

There are many factors that influence the reliability of visual inspection.  These factors fall into 5 different category according to the report from Judi E See: “Visual Inspection: A review of the literature”.  The table below is taken from the report:

This is true for visual inspection.    The Federal Aviation Administration (FAA) defines visual inspection as following:

 “The process of using the unaided eye, alone or in conjunction with various aids, as the sensing mechanism from which judgments may be made about the condition of a unit to be inspected.”

Note that if you have only an efficiency of 80% even The 4-eye principle where 2 people do the same inspection will only have an efficiency of 96%. The first inspector will detect 80% of the defects and let 20% pass.  The second inspector will detect 80% of the 20% that passed, thus 16% extra.

So should we abandon visual inspection?

Of course not. Visual inspection can give you a lot of information. But you should not implement visual inspection because it is the easiest solution. 

First of all you should try to design products which have a  high level of quality. You can use Design for six sigma (DFSS) for this.

Quality issues during the production should be tackled with structural problem solving methods such as  A3, 8D, PDCA, …

Try to limit quality controls where the judgement of an inspector is needed to a minimum. Use Poka Yoke solutions and (automated) inspection equipment were possible.

If you have done all of the above you can implement a visual inspection step.