Sampling Techniques

Introduction

Sampling plans in the form of published standards are used in industry world-wide as a means of assisting quality assurance personnel to decide whether to accept or reject lots of product presented for inspection.

There are a number of standards available. Commonly used standards are ANSI/ASQ Z1.4 (replacing MIL-STD-105E) for inspection by attributes and ANSI/ASQZ1.9 and for inspection by variables. The standards provide inspectors with information on the size of sample to be selected and a set of rules and procedures, related to the AQL (acceptable quality limit), which enables the inspector, having examined the sample, to decide on the acceptance or rejection of the lot.

The sampling standards provide organizations with an effective and cost efficient means of selecting sample sizes and devising sampling plans. The switching rules, when correctly used, provide for reduced sampling when incoming product is of consistently good quality, thereby considerably reducing inspection costs, while also providing for more stringent tests should quality levels decline.

The variables sampling plans (e.g. ANSI/ASQ Z1.9) should be more widely used, in circumstances where samples are examined by measurement, because inspection by variables requires considerably smaller sample sizes, when compared to inspecting the same characteristic by attributes. Emphasis will be placed in this training course on the correct use of the switching rules and selection of the appropriate standard. The sampling standards ANSI/ASQ Z 1.4 and ANSI/ASQ Z 1.9 will be used for reference on this training course.

Objective

1. On completing this course the participant will:

• Understand how to use the sampling standards for inspection  by attributes and variables
• Know how to select the appropriate sample size and how to apply the rules for acceptance or rejection of and inspection lot
• Know when to use normal, tightened and reduced sampling plans

2. Choosing a Sampling Plan with a given OC Curve

• Understand the basic statistics underlying the sampling plans and    the four graphs explaining about a sampling plan

3. Understand sampling inspection terminology

4. Understand α-error and β-error

Methodology

Lecture, Discussion & Case Studies

Course Content

1. Introduction to the statistics relevant to inspection by attributes and variables

2. Explanation of sampling inspection system terminology including:

• Acceptance quality limit (AQL), inspection levels, attributes, variables, probability of acceptance, operating characteristic (OC), random sampling, consumer’s risk , producer’s risk and lot tolerance percent defective (LTPD).

3. The correct approach to determining the acceptance quality limit (AQL)

4. Creating OC Curve(OC),Average Outgoing Quality Curve (AOQ),Average Total Inspection Curve

    (ATI) and Average Sample Number Curve (ASN) using excel

5. Identifying the appropriate inspection level and sample sizes in the sampling tables

6. Exercises in the use of the sampling tables selecting the sampling plans for inspection by attributes        and variables.  How the tables are used to decide on acceptance or rejection of a lot.

7. Explain the statistics underlying sampling plans, describes in detail how the sampling tables for both

    attributes and variables are  used

8. How to correctly use the switching rules for optimum effectiveness

Target Audience

• Managing Director / Factory Manager/Production Manager/Supervisor
• QA Managers/ QA Executive/ Supervisor
• QC Manager , Engineering Managers, Design Engineers, Quality  Engineers and Quality Auditors
• Staff who have responsibility for undertaking inspection of incoming  or outgoing lots of product