DPPM Calculation

Chart, graph, sampling, plan, calculation, z1.4

Question

Recently, there is a debate in my organization about Defective Parts Per Million (DPPM) computation.
Camp 1 – DPPM = (No of parts rejected / No of parts inspected) * 1,000,000
Camp 2 – DPPM = (No of parts rejected / No of parts received) * 1,000,000
We perform sampling inspection based on AQL.
Camp 1 insists they are correct and likewise for Camp 2.  Which is correct or more appropriate to reflect supplier quality?

Answer

This is not an uncommon question. If you look at the standard, they define the % nonconforming as the number of parts nonconforming/number of parts inspected x 100. If you are looking at DPPM, instead of multiplying by 100, you put in 1,000,000. This means that by your definition, Camp 1 is correct. This is also what was intended by the creators of the sampling scheme.

Jim Bossert
Sr Performance Improvement Specialist
JPS Hospital
ASQ Fellow, CQE, CQA, CMQ/OE, CSSBB, CSSMBB
Fort Worth, TX

Combating contamination

Q: We want to ensure that we are receiving clean containers to package our products. How can we improve our incoming inspection process?

A: You should encourage your vendor to ship only clean containers. Then, be sure that the shipping and receiving process doesn’t cause contamination. If you can determine the source or sources of the contamination, the best fix is to remove the cause.

If that approach is not possible and you have incoming containers that may have some contamination, then consider the following elements in creating an efficient incoming inspection process.

1) How do you detect the contamination?

Apparently, you are able detect the container contamination prior to filling them, or are able to detect the effect of the contamination on the final product. Given that you are interested in creating an incoming test, let’s assume you have one or more ways to detect faulty units.

As you may already know, there are many ways to detect contamination. Some are faster than others, and some are non-destructive. Ideally, a quick non-destructive test would permit you to inspect every unit and to divert faulty units to a cleaning process. If the testing has to be destructive, then you’ll have to consider lot sampling of some sort.

There are many testing options. One is the optical inspection technique, which may find gross discoloration or large debris effectively. Avoid using human inspectors unless it’s only a short term solution, as we humans are pretty poor visual inspectors.

Another approach is using light to illuminate the contamination, such as a black light (UVA). Depending on the nature and properties of the contamination, you may be able to find a suitable light to quickly spot units with problems.

Another approach, which is more time consuming, is conducting a chemical swab or solution rinse and a chemical analysis to find evidence of contamination. If the contamination is volatile, you might be able to use air to “rinse” the unit and conduct the analysis. This chemical approach may require specialized equipment. Depending on how fast the testing occurs, this approach may or may not be suitable for 100 percent screening.

There may be other approaches for detecting the faulty units, yet without more information about the nature and variety of contamination, it’s difficult to make a recommendation. Ideally, a very fast, effective and non-destructive inspection method is preferred over a slow, error prone, and destructive approach. Cost is also a consideration, since any testing will increase the production costs. Finding the right balance around these considerations is highly dependent on the nature of the issue, cost of failure, and local resources.

2) How many units do you have to inspect?

Ideally, the sample size is zero as you would first find and eliminate the source of the problem. If that is not possible or practical, then 100 percent inspection using a quick, inexpensive, and effective method permits you to avoid uncertainties with sampling.

If the inspection method requires lot sampling, then all of the basic lot sampling guidelines apply. There are many references available that will assist you in the selection of an appropriate sampling plan based on your desired sampling risk tolerance levels.

Another consideration is the percentage of contaminated units per lot. If there is a consistent low failure rate per lot, then lot sampling may require relatively large amounts of tested units. You’ll have to determine the level of bad units permitted to pass through to production. Short of 100 percent sampling, it’s difficult (and expensive) to find very low percentages of “bad” units in a lot using destructive testing.

3) Work to remove original source(s) of contamination to permit you to stop inspections.

I stress this approach because it’s the most cost effective in nearly all cases. In my opinion, incoming inspection should be stopped as soon as possible since the process to create, ship and receive components should not introduce contamination and require incoming inspection to “sort” the good from the bad.

Fred Schenkelberg
Voting member of U.S. TAG to ISO/TC 56 on Reliability
Voting member of U.S. TAG to ISO/TC 69 on Applications of Statistical Methods
Reliability Engineering and Management Consultant
FMS Reliability
www.fmsreliability.com

Related Resources:

Digging for the Root Cause, Six Sigma Forum Magazine, open access

Many Six Sigma practitioners use the term “root cause” without a clear concept of its larger meaning, and similar situations occur in Six Sigma training programs. As a result, many practitioners overlook root causes. Read more.

The Bug and the Slurry: Bacterial Control in Aqueous Products, ASQ Knowledge Center Case Study, open access

When a customer reported a problem using the polycrystalline diamond (PCD) slurry supplied by Warren/Amplex, the company traced its product through the supply chain in order to identify the cause and quickly implement a solution. Read more.

Explore the ASQ Knowledge Center for more case studies, articles, benchmarking reports, and more.

Browse articles from ASQ magazines and journals here.

Dock to Stock

Suppliers, supplier management

Q: I have been tasked with implementing a dock to stock policy. Does an expert have any advice or information to share towards forming a dock to stock policy?

A: To begin, here is a brief definition of dock to stock (DTS):

Dock to stock is a receiving method whereby materials are delivered directly to point of use (storage or manufacturing), skipping the normal receiving inspection.

For most organizations, parts which are given a DTS status are those which have been “proven” to be compliant. It is common practice to perform a receiving inspection on the parts for a minimum of five deliveries (some companies choose 10).

After a supplier has proven to deliver a compliant product five times, that individual item/part number is given DTS status. It is then general practice for production/assembly departments or line personnel to verify compliance as needed. If a product is found to be noncompliant, it is put on a contingency list and must prove its validity again — usually through five to 10 compliant shipments before it is returned to DTS status.

Keep in mind that the DTS process is rarely used in some industries/companies. For example, a company certified to ISO 13485 (medical devices) would not use DTS due to FDA regulations — here’s an excerpt from 21 CFR 820.80 (b):

“Receiving Acceptance Activities: Incoming product shall be inspected, tested or otherwise verified as conforming to specified requirements.”

In short, determining how many acceptable shipments to qualify a supplier for DTS status is up to the company. Requesting a certificate of compliance with each shipment can tend to encourage a supplier to ensure their own quality, as does a yearly audit of the supplier’s facilities (if appropriate).

I hope using the guidelines above will help lead you toward your goal.

Bud Salsbury
ASQ Senior Member, CQT, CQI

Related Content:

Browse the free, open access resources below, or find more in the ASQ Knowledge Center.

Chinese OEM Reduces Returns With Improved Product Testing, ASQ Knowledge Center case study

When Continental Automotive Systems, Tianjin, China, began producing an electronic component known as the silver box, the return rate was more than 1,200 parts per million (ppm), versus a goal of less than 100 ppm. A Six Sigma improvement team used quality tools including trend charts, Pareto charts, and cause-and-effect diagrams to analyze the failure modes for the reported defects, finding that many were not being covered by product testing processes. Read more.

Cost-Effectiveness Based Performance Evaluation for Suppliers and Operations, Quality Management Journal

This research establishes a cost-effectiveness based  performance evaluation system for suppliers and operations. The purpose is to provide a methodology for “integrating supplier and manufacturer capabilities through a common  goal, profitability improvement, based on lowering the cost of purchased materials.”  Read more.

Expert Answers: Stock and Standards, Quality Progress

The advisability of implementing dock-to-stock is discussed. Read more. 

Ask A Librarian

ISO 2859-3 Skip-lot Sampling 5.1.1, 5.2.1

Suppliers, supplier management

Q: Our quality team is trying to improve inspection efficiency and enhance supplier management by employing ISO 2859-3:2005 Sampling procedures for inspection by attributes — Part 3: Skip-lot sampling procedures.

Here are two questions on product qualification related to clause 5.2.1 Generic requirements for product qualification.

1. The standard requires that:

b) The product shall not have any critical classes of nonconforming items or nonconformities.

First, my understanding is that the risk level with any potential failure or nonconforming of the product should be low to customer — is this correct? Second, if a candidate product carries some critical features (dimension of mechanical product), but also carries a number of low risk features, can we apply the skip lot concept only to the non-critical features? And continue to perform lot-by-lot inspection with critical features? We are concerned the definition of “product” in the standard is a generic term and could be interpreted as feature of a physical product.

2. The standard requires that:

c) The specified AQL(s) shall be at least 0,025 %.

Does this mean the AQL value should be less than or greater than 0.025%? I assume “greater.” In our company, the most often used is AQL 1.0 and AQL 2.5, which I think meets the requirement.

We would greatly appreciate your help.

A: My name is Dean Neubauer and I am the U.S. Lead Delegate to Subcommittee 5 on Acceptance Sampling and co-author of Acceptance Sampling in Quality Control, 2nd edition., 2009. CRC Press. I hope I can help you.

Let’s start with question 1.

The general idea of skip-lot sampling is to reduce the number of times incoming lots inspected due to exceptional quality on behalf of the supplier.  ISO 2859-3 states this in the beginning as:

The purpose of these procedures is to provide a way of reducing the inspection effort on products of high quality submitted by a supplier who has a satisfactory quality assurance system and effective quality controls.

The reduction in inspection effort is achieved by determining at random, with a specified probability, whether a lot presented for inspection will be accepted without inspection.

A skip-lot sampling plan is also known as a cumulative results plan.  In general, such plans require certain assumptions to be met regarding the nature of the inspection process:

  • The lot should be one of a continuing series of lots
  • We expect these lots to be of the same quality
  • The consumer should not expect that any lot is any worse than any of the immediately preceding lots
  • The consumer must have confidence in the supplier not to pass a substandard lot even though other lots are of acceptable quality

Under these conditions, we can use the record of previous inspections as a means of reducing the amount of inspection on any given lot.  ISO 2859-3 states the above in 5.1.1 Requirements for supplier qualification:

The requirements for supplier qualification are as follows.

a) The supplier shall have implemented and maintained a documented system for controlling product quality and design changes. It is assumed that the system includes inspection by the supplier of each lot produced and the recording of inspection results.

b) The supplier shall have instituted a system that is capable of detecting and correcting shifts in quality levels and monitoring process changes that may adversely affect quality. The supplier’s personnel responsible for the application of the system shall demonstrate a clear understanding of the applicable standards, systems and procedures to be followed.

c) The supplier shall not have experienced any change that might adversely affect quality.

The underlying assumption here is that the supplier quality is exceptional (low nonconforming level).  The skip-lot plan is applied to each characteristic, or feature, separately.  If several characteristics are present, then try to test for at least one of them.  In your situation, if you have critical characteristics you should not be doing skip-lot inspection due to the risk of ignoring (skipping) a potentially dangerous lot.  On the other hand, you can use a skip-lot plans for non-critical (major and minor) nonconformities and they will have different AQL levels associated with them.  Your listing of AQLs of 1.0% and 2.5% are typical for major and minor nonconformities.  Critical defects will typically have an AQL less than 1.0%, such as 0.25% to 0.65% (0% is preferred but theoretically unattainable as you would have to do a perfect 100% inspection, i.e., no inspection error).

The subclause referenced in question 2 states that the AQL level must be greater than or equal to 0.025%. Your levels of 1.0% and 2.5% can be used.

Dean Neubauer

U.S. Lead delegate for Subcommittee 5 on Acceptance Sampling on ISO Technical Committee 69 on Applications of Statistical Methods.