TE Taylor Enterprises, Inc.
www.variation.com
Quality and Statistics
Books, Software, Training and Consulting


Search variation.com

Enter keywords: 

Exact Match Search

 

Site Map

Products (HOME)

Books

Software

Courses

Consulting


Expertise

Acceptance Sampling

Process Validation

CAPAs and Trending of Quality Data

FMEA

Measurement Systems Analysis

Spec Setting, Tolerance Analysis and Robust Design

General Statistics

Statistical Process Control

Design of Experiments

Six Sigma


Store  

What's New

Technical Library

FAQ


  Contact Info

Chairman
Dr. Wayne A. Taylor
President
Ann Taylor
Telephone
1 (847) 367-1032
FAX
1 (847) 367-1037
Postal address
5510 Fairmont Rd.
Libertyville, IL 60048
USA
Electronic mail
info@variation.com
Web
www.variation.com

 

Subscribe to our Web Site

By entering your e-mail address and clicking the Subscribe button, you will automatically be added to our mailing list.  You will receive an e-mail when new versions of our software or books are available as well as other significant announcements.  (privacy policy).

E-mail address to send notifications to:

    

 

Classifying Defects and Selecting AQLs

Dr. Wayne A. Taylor

It is common practice to classify defects accordingly to their severity and then to assign each category an acceptable quality level (AQL). How justified is this practice and what guidelines can be given in assigning AQLs? The answer to this depends on how AQL is interpreted. Most generally the AQL is viewed as kind of a cut off point. Lots up to the AQL are acceptable for release. Any lot worse than the AQL should be rejected. This does not mean that all such lots are actually rejected. Often the sampling plan is only capable of rejecting lots significantly above the AQL. However, if a sampling plan rejects a 2% defective lot when the AQL is 1%, no one would consider that an error.

This interpretation of the AQL closely coincides with what is called the break even quality. The break even quality is that point where the cost of 100% inspecting a lot is exactly equal to the benefits obtained as a result of fewer defects being released. Lots free of defects (0% defective) are best released. 100% inspecting such lots is a waste of time and money. On the other hand, lots with high levels of defects should be 100% inspected. The cost of the 100% inspection is small compared to the consequences of the defects it eliminates. Somewhere between 0% defective and 100% defective, there is a point where it is a tie.This point is called the break even quality. Figure 1 shows this point graphically.

Figure 1: Break Even Quality

Figure 1: Break Even Quality

The break even quality depends on:

k1 = per unit cost of performing the 100% inspection.

k2 = loss incurred by customer and others as the result of the release of a single defective unit.

The break even quality (BEQ) is then:

Suppose that the per unit cost of inspection is $0.10 and that each defective released results in an average loss of $10. Then the break even quality is 1% defective.

If product AQLs are viewed as break even qualities, then the above formula can be used to establish AQLs. Defects are commonly classified as critical, major, and minors. These categories correspond to differ severity's, i.e., different values of k2. Within a category, the defects have similar consequences. For example, major defects render the product nonfunctional. The customer is little concerned with why it doesn't work. The impact is the same. Therefore it makes sense to group these defects together if their inspection costs are similar. In particular, visual defects should be grouped together. Functional defects with higher costs of testing should be inspected separately, not only using a separate sampling plan, but also with a higher AQL.

One final note is in order. The AQL is a guide as to what to do with the lot once it is produced. For example when the AQL is 1%, a 0.5% defective lot is best released. However, it would be better still to have produced a lot free of defects. The AQL should not be viewed as a production goal or as permission to produce defects.

Appeared in FDC Control, Food Drug & Cosmetic Division ASQC, No. 103, November 1993, p. 4

Copyright 1993 Taylor Enterprises, Inc.


Copyright 1997-2012 Taylor Enterprises, Inc.
Last modified: August 04, 2012