My order is pretty large and I don’t have the time or budget to inspect it 100%, how can I determine what is a statistically reliable sample size?

The AQL chart was invented for just that situation. The standard definition of Acceptance Quality Limit (AQL) is “the maximum defective percent (or the maximum number of defects per hundred units) that, for purpose of sampling inspection, can be considered satisfactory as a process average”.

Basically, select a sampling size quantity (based on the AQL tables) to be inspected, and after inspection, according to the number of defects found (critical, major, minor defects), you will know if you can accept or reject your shipment. Let me explain, first determine the sampling size according to your ordered quantity and your level of severity (I, II or III). The standard level, the one used by default and by 98% of the people is the level II for a standard inspection.

Let’s take a hypothetical inspection of a 30 000 pieces lot. The Table A, under the column level II, gives us the letter M, which corresponds in the table B to a sampling size of 315 pieces to inspect.
The usual AQL used by people is generally: 0 / 2.5 / 4 for Critical / Major / Minor defects. For the letter M, 315 pieces to inspect, the column 2.5 for defects gives us two numbers: 14 & 15.

Thus Among a sampling size of 315 pieces inspected, the maximum number of major defects authorized is 14. If we find 15 major defects, you should reject your shipment and / or perform defect sorting, ask for a re-work of the production or ask for a replacement of your production. Then you should get your shipment re-inspected a second time.

In the contrary, for these 315 pieces inspected, if we find less than 15 defective items, this means that there are 95% chances to have less than 5% (15/315) defects in the whole inspected lot.

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