Sampling Plan and Quality Control

Lot quality assurance sampling (LQAS) is a random sampling methodology, originally developed in the 1920s as a method of quality control in industrial production. Compared to similar sampling techniques like stratified and cluster sampling, LQAS provides less information but often requires substantially smaller sample sizes.

Sampling comprises the operations designed to select a portion of a pharmaceutical product (for definition, see glossary) for a defined purpose. The sampling procedure should be appropriate to the purpose of sampling, to the type of controls intended to be applied to the samples and to the material to be sampled. The procedure should be described in writing. All operations related to sampling should be performed with care, using proper equipment and tools. Any contamination of the sample by dust or other foreign material is liable to jeopardize the validity of the subsequent analyses.

Sample A portion of a material collected according to a defined sampling procedure. The size of any sample should be sufficient to allow all anticipated test procedures to be carried out, including all repetitions and retention samples. If the quantity of material available is not sufficient for the intended analyses and for the retention samples, the inspector should record that the sampled material is the available sample (see Sampling record) and the evaluation of the results should take account of the limitations that arise from the insufficient sample size.

The most commonly used is the single-stage sampling plan by attributes.

What does it mean, and what are the other options?

A “single-stage sampling plan” dictates that a certain number of pieces (n) should be drawn and inspected. That number n depends mainly on the size of the batch (and also on the inspection level). If the number of defects is under the AQL limit, the result is passed.

In a “double-stage sampling plan“, the inspector would start by taking a smaller number of sample (n1). If the number of defects in n1 is above certain limits, more samples are picked.

An “accept on zero” plan is a weird animal that is usually not a good idea in China. Some importers, who are sensitive to legal litigation by their customers, accept batches only if no defective unit is found. The only advantage is the fewer samples need to be checked. More in-depth information here.

A “sampling plan by attributes” classifies the samples as either “non defective” or “defective”. There is nothing in between.

A “sampling plan by variables” allows for a finer evaluation. For example, the length of the product is measured, and the exact findings are taken into account when a decision is made.

A “rectifying sampling plan” is applicable if the defects that are found can be corrected immediately. It is not very different from a sampling plan by attributes, but it takes into account that the batch is of higher quality after the inspection… and, in case of inspection failure, the whole batch should be inspected.

“Continuous sampling” is the best plan when products are made individually in a continuous flow. It makes no sense to pick samples inside each “batch”. It consists of several phases:

At the beginning, each piece is checked (that’s the “screening”).

After a certain number of pieces were found satisfactory, only certain pieces are checked randomly (that’s the “sampling”).
If a piece is defective: back to screening.

Scroll Up