Sampling According to the MIL-STD-105 Standard

In quality control, inspecting 100% of produced parts is often costly, time-consuming, or even impossible. That’s why sampling plans are used, they allow a small, representative portion of a lot to be inspected to estimate overall quality. One of the most recognized standards in this field is MIL-STD-105, also known as Acceptable Quality Level (AQL) sampling.

What is MIL-STD-105?

MIL-STD-105 is a U.S. military standard (officially replaced by ANSI/ASQ Z1.4, but still widely used under its original name) that defines attribute sampling plans. It allows a decision to be made, based on a limited number of inspected units, whether to accept or reject an entire lot.

Key Elements of the Standard:

AQL (Acceptable Quality Level): The AQL represents the maximum percentage of defective items considered acceptable in a production lot. For example, an AQL of 1.5 means up to 1.5% of non-conforming parts can be tolerated.

Lot size: This is the total number of units produced in a given batch. The larger the lot, the more representative the sample will be.

Severity:

        –       Normal: Standard inspection, used in routine situations.

        –       Tightened: Applied when there are doubts or a history of quality issues.

        –       Reduced: Used when consistent quality has been demonstrated over time.

How Does the Process Work?

        1.     Identify the lot size

        2.     Choose the desired AQL level

        3.     Determine the sample size using the MIL-STD-105 table

        4.     Count the number of defects in the sampled units

        5.     Compare the result to acceptance criteria:

        –       If the number of defects is equal to or below the acceptance threshold, the lot is accepted.

        –       If it exceeds the threshold, the lot is rejected (or subject to additional action).

Follow-up Actions in Case of Lot Rejection

Lot Segregation:

The lot is quarantined to prevent defective parts from being used in production.

Possible Actions:

        –       100% Inspection: Every part is checked individually to isolate conforming items.

        –       Return to Supplier: The lot is sent back with a non-conformity report.

        –       Rework / Reconditioning: Defective parts are repaired if possible, then re-inspected.

Enhanced quality monitoring:

The following measures are applied:

        –       Larger sample size: More parts are inspected during each quality check.

        –       Stricter acceptance criteria: Fewer defects are tolerated in the sample.

Single sampling plan – normal inspection