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Determining Your Sample Size Per ANSI ASQ Z1.4 AQL Table PAGE 1 OF 12 EFFECTIVE: 9/30/
1. LOT SIZE
On the far left of the first side of the ANSI Z1.4 table, you will see a section for “lot size”, referring to your total order quantity. Lot size is your starting point for using the ANSI Z1.4 table, represented by a range of order quantities expressed in units.
This part of using the AQL sampling plan tables is easy— simply choose the lot size range containing your total order quantity. For instance, let us say you have an order quantity of 300 parts. You would use the lot size range of 281 to 500 units.
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2. INSPECTION LEVEL
To the right of the lot sizes, you will see the ANSI Z1.4 table is further divided into two categories, “general inspection levels” and “special inspection levels”. The inspection level you choose is a critical determinant of your ANSI sampling plan and the scope of your product inspection. PTI Technologies requires inspection level II for the sample inspection report per QA19A.
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The second part of the AQL sampling tables then lists the sample size, or the number of units to pull for inspection, next to the corresponding code letter. For H, your initial sample size will be 50 units. This is just a starting point.
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You will notice there are also arrows pointing up or down for certain sample sizes under each of the acceptable quality levels. These arrows indicate where you should use a different sample size based on your tolerance for defects, or your AQL, rather than your lot size.
The sample size determined by your lot size can sometimes be too large or too small for your AQL. You may require a larger sample size if you choose a relatively low AQL, for instance. The lower your tolerance for defects, the more units you will need to inspect to reasonably predict the average quality of the total order. In addition, if the appropriate sample size equals or exceeds your lot size, you will inspect 100 percent of the order.
A large sample size is often redundant for large lot sizes if you have a higher tolerance for quality defects. You can get the same confidence in results from inspecting a smaller sample size, which will be less time consuming and less expensive. In these cases, follow the arrows to the appropriate sample size and use this one for your inspection. Shown in example 1.
Returning to the above example, PTI requires an AQL of 0.65 for major defects. You will need to use a sample size of 80 units (coded as J), instead of the 50-unit sample size corresponding to H, as shown by the downward arrow.
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Example 1
In this example, we will use a lot or batch size of 200. In this instance, your code letter will be “G”
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Looking at table II, you will see your starting sample size is 32
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Example 2
In this example, we will use a lot or batch size of 18. In this instance, your code letter will be “C”
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Our starting sample size is 5.
