The Inspection Levels
Three general and four special inspection levels are commonly used. The general inspection levels (1 to 3) are typically used for non-destructive inspection. Level 2 is considered the norm (except for small sample sizes). Level 1 requires only 40 percent of inspection level 2 and can be used where less discrimination is needed. Level 3 equals 160 percent of the amount of inspection level 2. Level 3 will give a lower risk of accepting a lot with excessive number of defects. However, inspection of a larger number of samples is required.
Special Levels S-1, S-2, S-3 and S-4 may be used where relatively small sample sizes are necessary or large sampling risks can be taken. Examples of this are inspections involving destructive or costly (time consuming) type inspections where large lots are involved, small sample sizes desired, and large risks can be tolerated such as repetitive processes (screw machine, stamping, bolting operation, etc.) performed by a quality supplier. Larger sample sizes are required for inspection levels increasing from S-1 to S-4.
Selecting a Sampling Plan
For a hypothetical total shipment quantiy of 1500 units, you first look at Table-1 below, on the left side under “Lot or Batch Size”. You will find that the number 1500 falls within the specified 1201-3000 range. Then move across the horizontal access to the right and stop under the “General Inspection Levels II” column where you will see the letter “K”. Now proceed to Table-2 below Table-1 and scroll down the far left column “Sample Size Code Letter” to the “K” position. The very next column towards the right is “Sample Size” which in this case indicates the sample lot size to be 125 units. Now move across the horizontal axis toward the right until you arrive at the column of the desired sampling strigency level. On the far left 0.065 represents the most stringent AQL level and 6.5 on the far right represents the most lenient AQL level meaning more defective units are allowed.
Note: The “Ac” means the maximum number of defective units with which the lot can still be considered accepted. The “Re” represents the number at which the sampled lot is to be rejected. KRT Audit Corporation inspects and reports back to you using two stringency levels, a normal and a tightened level. And don’t worry, these are just guidelines which you as the buyer can overrule so long as your supplier agrees to your different set of guidelines and stringency levels.