Title: Sampling Plans
Course: PW-Course-Import-Test > Section: Module 4: Sampling and Calibration

Module 4

There are two types of statistical sampling plans used to perform inspection. Selection of these plans depends on quality characteristics of the data.

- Attribute Sampling Plan - This plan is used to provide information to determine whether to accept or reject material that is already produced based on less than 100% inspection. This is the most common type of sampling plan used in the medical device industry. For the purpose of this class we will mainly focus on the attribute sampling plan. ANSI/ASQ Z1.4 is the sampling table used for a attribute sampling plan.

- Variable Sampling Plan - This plan is used when quality is measurable on a continuous scale. This plan samples the process when the part or product is being built.  ANSI/ASQ Z1.9 is the sampling table used for a variable sampling plan.

Sampling Inspection by Attributes (ANSI/ASQ Z1.4)ANSI/ASQ Z1.4 table is the most commonly used table and is used to perform attribute sampling. In this type of sampling, individual items in a random sample are inspected and are judged as acceptable or not acceptable. In order to use the  ANSI/ASQ Z1.4 table, the following information is needed. A quality inspector is given this information before they consult the table for sample size.

- Lot Size - For receiving inspection this the quantity of components sent by the supplier. For inprocess inspection this is the quantity of product being manufactured.
- Inspection Level - There are seven inspection levels I, II, III, S-1, S-2, S-3 and S-4. Level II is the default level.
- AQL - There are 26 AQL levels ranging from 0.000 to 1000. Level 4.0 is most common.
- Type of Sampling - There are single, double or multiple sampling. Single sampling is most common.

This video gives step-by-step instructions on how to use the ANSI/ASQ Z1.4 table (the PowerPoints used in the presentation). I have found this very useful in the understanding of the use of this tables. The video walks you through single and double sampling plans. Both these plans are actively used in the industry. The engineer predetermines what plan to use for a particular component; the quality inspector does not have to make this decision.

Walk through the two examples provided in video/PowerPoints. It will provide you a good idea of how these tables are used. Later on in this module, we will determine sample sizes using a couple of examples of our own.

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How to read the ANSI tables for inspections based on random sampling

Many a times the sampling plan and acceptance criteria are provided by the engineer in the drawing. This is usually done in a form of a table. When such a table is present, the quality inspector does not have to refer to any sampling plan. All they need to do is know what the lot size is and the table will tell you the quantity of parts to sample and acceptance criteria. Now that makes it very easy. Lets look at an example to better understand this.

Say, for example, you have received 30,000 metal rings that go into building a medical device. The drawing of this ring lists the specifications that need to be measured. Along with these specifications is a table that lists the sample size requirement for different lot quantities. Such a table would look as follows:

Quality Received | Sample Size | Accept | Reject
10,000 | 200 | 7 | 8
20,000 | 315 | 10 | 11
30,000 | 315 | 10 | 11
40,000 | 500 | 14 | 15

Before constructing this table the engineer knows that the metal rings are always ordered and received in quantities of 10,000, 20,000, 30,000 or 40,000.  The engineer has used a general inspection level and AQL 1.5 for the sample size and accept-reject criteria.

So for a lot of 30,000 rings that you have received from the supplier you will randomly inspect 315 parts. You will accept if you find 10 bad parts but will reject the entire lot if you find 11 bad parts out of the 315 you inspected.

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Basics of Attribute Sampling