Lesson 2: Statistical Sampling - DAU€¦ · Web viewSampling schemes include a combination of...

CMQ101 Government Contract Quality Assurance Fundamentals – Module 4, Lesson 2: Statistical Sampling

Lesson 2: Statistical Sampling

Content Instructor Notes

1. INTRODUCE the lesson, Statistical Sampling.

TELL students that this lesson covers:

DCMA policy on sampling Types of sampling techniques Zero-based sampling plan process Normal – reduced – tightened inspection levels

2. READ the lesson Terminal Learning Objective (TLO) as listed on screen.

3. READ the Enabling Learning Objectives (ELOs) as listed on screen.

4. EXPLAIN that these are the lesson topics.

1 Defense Acquisition UniversityInstructor Support Package (ISP)Instructor Manual




5. POINT OUT why this lesson is important:

Sampling allows the QAS, to maximize limited time. Sampling allows the QAS to accept large quantities

while minimizing the risk of accepting non-conforming products that would negatively affect the warfighter’s mission and success.

6. EXPLAIN that DCMA policy uses:

Zero-based sampling. Random sampling techniques. Statistically valid sampling plans.

EMPHASIZE that suppliers may have other sampling requirements, and the QAS must ensure suppliers meet their requirements and that the QAS can verify.

POINT OUT that the QAS should make sure that the supplier understands and is using statistically valid sampling plans. The QAS should verify the supplier’s plan as part of the Standard Inspection checklist, which is in the Performance Factors for All Suppliers tab of the Risk Profile and Plan Tool.

7. DEFINE sampling system as a collection of sampling schemes.

MENTION examples to include:

ANSI/ASQ Z1.4-2008. MIL-STD-1916. Zero Acceptance Number Sampling Plans.These Sampling Systems are discussed in detail in CMQ200.

EMPHASIZE that DCMA policy states that the QAS will use zero-based sampling plans unless something else is called-out by the customer in a QALI.

POINT OUT the difference between ANSI/ASQ Z1.4, MIL-STD-1916, and zero-based sampling systems.

ANSI/ASQ Z1.4 – lots can be accepted on one





defect and rejected on two. Lots can be accepted based on specific accept/reject criteria, but the supplier is required to remove/replace defective product prior to shipment. Suppliers are NEVER allowed to knowingly ship defective product.

Zero-based – lots are rejected on one defect





8. DEFINE key terms:

Sampling schemes include a combination of with switching rules and a provision for discontinuance of inspection.

Individual sampling plan is that specific plan that states sample size and acceptance criteria.

9. DEFINE key terms related to examination:

An attribute is a characteristic that’s either go/no- go, pass/fail, good, or bad – not the actual measurement.

Characteristics or features could be dimensions, physical or chemical properties, or color or weight that are typically defined by a specification, standard or drawing.

Defect is a nonconforming characteristic of a part. Defective is a product containing one or more

defects. Nonconformity is a unit of product that contains one

or more defects.

10. DEFINE key terms related to submission of product:

When defining a “lot,” some process specifications clearly define formation of lots for inspection purposes. For example, MIL-STD-1916 paragraph 4.2.2, Formation and identification of lots or batches, states that the product shall be assembled into identifiable lots, sub-lots, or batches, or in such other manner as may be prescribed. Each lot or batch shall, as far as practicable, consist of units of product of a single type, grade, class, size, and composition, manufactured under essentially the same conditions, and at essentially the same time (normally a single shift/day).

Lot formations (homogeneity) are very important for evaluating supplier sampling plans. It is DCMA’s responsibility as QA personnel to ensure the supplier is adhering to the contract requirements.





EMPHASIZE that an inspection lot is not always a shipping lot: inspection lot must be homogenous.





11. PRESENT the AQL chart:

AQL is the acceptable quality level. It is related directly to the sample size based on the lot size, and it is the amount of risk that the Government is willing to take with the sample.

The process average, which is the percent nonconforming, is equal to the number of nonconforming divided by the number of units inspected and multiplied by 100. The product examination sheet automatically calculates that number.

EMPHASIZE the AQL is the worst tolerable process average the customer is willing to accept.

12. INTRODUCE Topic, Importance of Sampling to QA.

13. DEFINE sample.

A portion of a population that is representative of the entire population. A sample subset of the population.

One or more units of product are drawn from a lot or batch at random, without regard to their quality.

Number of units or product in the sample is the sample size.

Statistical sampling techniques can be used extensively by suppliers within their quality program or inspection system, unless otherwise directed by contract.





14. DEFINE acceptance sampling.

Used for the purpose of making accept/reject decisions on an entire lot or batch.

Used by the supplier as well as the Government.

15.

ASK the question: Why should the QAS sample?

DISCUSS answers:

Saves time. Saves money. 100% inspection is not always possible or accurate.

Remind students of the “Find the F” exercise in the Product Examination lesson.

Customer has requested it.POINT OUT that it is often impossible or impractical for the QAS to inspect 100% of the product. Sampling is used as an efficient and effective way to assure that the supplier and its product is in compliance with the contract.

16. ILLUSTRATE random sampling procedure:

Every item in that population has an equal opportunity to be selected

If the sample taken is not representative of the population, it is a biased sample that can lead to misleading results.

Random sample can be drawn in several ways:o Serialized items can be randomized by the

S/Ns or items without serial numbers can be assigned numbers.

o For bulk material a numbered area or zone can be used to lay the material out. Samples can be drawn from numbered zones located by using a random number generator.





ASK the question: What is the answer if a supplier comes to the QAS and says, “We are almost done with this lot, only one more skid to go. Can you go ahead and take your sample from this?”

DISCUSS Answer “No,” because every item has not had the equal opportunity to be selected in the sample.





17. EXPLAIN zero-based sampling plans

Since 1983, DCMA has used Zero based sampling plans to assure the supplier is meeting contract technical requirements.

“Zero-based” means that for the lot to be accepted there can be no defects discovered in the sample.

DCMA uses the C=0 sampling plan tables from the ASQ publication “Zero Acceptance Number Sampling Plans,” by Nicholas L. Squeglia.

ASK the question for discussion: When performing zero-based sampling and one defect is found in the first item inspected, what should happen? Reject the lot because a defect was found or finish the sample?

DISCUSS Answer: Finish the sample; if the sample is not finished, there is not a true picture of that lot. The QAS needs to be able to cite to the supplier everything that was found. Otherwise the lot will be rejected for that defect, and the supplier will screen the lot for that defect alone and then re-present it.

18. EXPLAIN the risks present when sampling.

Customer’s Risk (buying activity)/DCMA’s risk: nonconforming products will be accepted.

Producer’s risk/Supplier’s risk: customer/DCMA will not accept conforming products.

POINT OUT that this risk is minimized by using statistically valid plans.

19. INTRODUCE Next Topic, There are three types of Inspection under a Sampling Plan.





20. BRIEFLY SUMMARIZE bullets on screen.

21. EXPLAIN the three types of inspection:

Start with normal unless directed otherwise by customer.

Switch to reduced once confidence in supplier is established.

Switch to tightened if nonconformances are discovered.

POINT OUT that there are guidelines in Military and ANSI sampling plans for “switching.”

22. EXPLAIN switching rules between reduced, normal, and tightened inspection. DoD only uses the Single Sampling Plans, and acceptance is always based on zero nonconformities. The following are the rules as they apply to our product:

The supplier starts with Normal sampling.[Select screen to enlarge NORMAL circle.]

If it has 10 consecutive lots without nonconformities, and production is steady, and it’s approved, the supplier can go to Reduced sampling.[Select screen to enlarge REDUCED circle.]

The supplier can continue Reduced sampling until it has an unacceptable lot, or production becomes irregular, or there are other conditions to consider that warrant going back to Normal sampling.

If the supplier is performing Normal sampling, and it has two unacceptable lots out of the last five or fewer lots, it must go to Tightened sampling.[Select screen to enlarge TIGHTENED circle.]

The supplier will stay on Tightened sampling until:





o It has five consecutive acceptable lots. In which case, it can go back to Normal sampling; or

o While on Tightened sampling, it has five unacceptable lots (these do not have to be consecutive). In this case, discontinue inspection until the supplier takes effective corrective action to prevent future nonconformities. Once inspection resumes, it will be at the Tightened sampling level.

EMPHASIZE that when switching from normal to tightened or reduced inspection, the sample size changes but not the AQL.





23. INTRODUCE Topic, Zero-based Sampling.

24. INTRODUCE the DCMA internal process for zero-based sampling by BRIEFLY SUMMARIZING the bullets on screen.

In zero-based sampling, if one defect is found, the QAS should request corrective action from the supplier.

25. EXPLAIN that once the population and inspection & acceptance criteria are identified, the sample system and size is determined. The following three sample systems that are commonly used:

ANSI/ASQ Z1.4 2008, Sampling Procedures and Tables for Inspection by Attributes

MIL-STD-1916, DoD Preferred Procedures for Acceptance of Product

Zero-Based Acceptance Sampling

POINT OUT that the QAS cannot hold the supplier to a tighter standard that what the contract requires.





26. EXPLAIN that the QAS sample is independent of supplier’s sample.

REMIND students that they should be validating the supplier’s work.

POINT OUT AQL criteria:

.4% on all critical characteristics 1.0% on complex/critical products or DCMA-

identified significant characteristics 4.0% on non-complex/non-critical product

REMEMBER that once AQL and lot size is determined, sample size can be found.

27.

EMPHASIZE that sample size is dependent on lot formation.

Product Unit Identification makes it easier to perform randomization.

Lots may be created by:o Machine

o Operator

o Customer destination

POINT OUT that the QAS must know the processes and that unit identification is not needed in order to pull a random sample.

TELL students that random sampling tools are covered later in the lesson.

28. INTRODUCE DCMA’s zero-based plan policy:

Use the AQL table referenced in the DCMA policy to determine the correct sample size to pull.

POINT OUT that DCMA policy states:

For critical characteristics on critical safety items (CSIs), use an AQL of 0.40%. [Select screen to display arrow.]

For complex/critical products (excluding CSI and DCMA-identified significant characteristics) use an AQL of 1.0%. [Select screen to display arrow.]

For non-complex/non-critical products use an AQL





of 4.0%. [Select screen to display arrow.]





29. IDENTIFY the example.

Lot size of 25 AQL of 1.0

DEMONSTRATE finding the sample size on the AQL chart:

From the table, the left column is lot sizes the top row is AQL percentages.

Find the lot size; in this case it is 25. [Select screen to display 16-25 circled.]

Find the AQL which is 1.0%. [Select screen to display 1.0% circled.]

Follow the row over and the column down to where these intersect: that will be the sample size. [Select screen to display arrows from lot size and AQL.]

Find sample size: for this example, it is 13. Compare this to an AQL of .40 in which the sample size would be all 25 units or an AQL of 4.0 in which the sample size would be only seven units.

30. TELL students that the optional tool available to document Product Examination also contains a copy of the zero-based AQL chart that identifies sample size.

A sample is shown here.

EXPLAIN that when a lot size and AQL is entered the sample size is automatically highlighted.

[Select the button to open sample.]

DEMONSTRATE by entering in lot and AQL numbers.

PAUSE a moment to allow students to open the sample file and practice entering lot and AQL levels to see the sample size.

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33. INTRODUCE Topic, Generating Random Sample Numbers.

34. SUMMARIZE content.

POINT OUT that DCMA’s Product Examination Policy includes links to two spreadsheets, the “1711 with Random Number Generator” and the “Random Number Generator,” and a link to the Random.org web site.

35. INTRODUCE random.org, Random Integer Generator as another method to obtain random samples. [Select link to display.]

INFORM students that each item to be inspected must be numbered.

DEMONSTRATE process to use random.org:

Enter the total number of samples in the first field in Part 1. [Enter the sample number.]

Enter the serial number start and/or other identification numbers in the next two fields. [Enter a start and end number.]





Enter the number “1” in format columns. It doesn’t really matter what format is entered. [Enter 1 in the format field.]

Select the Get Numbers button. A list of random numbers appears. [Select the Get Numbers button.]

Select the Again! button if there are duplicate numbers. [Select the Again! button.]





36. PAUSE to allow students to practice using random.org.

Example:

Lot size of 1200. Sample size of 34.

37. INTRODUCE Topic, Initiating Acceptance and Non-Acceptance Activities.

38. SUMMARIZE bullets on screen.

39. EXPLAIN that once the sample is drawn:

Perform the examination using the inspection, test, witness, and verification techniques to assure that the product or item meets the stated technical requirements in the contract technical data package.

Use the correct accept/reject number from the sampling table.

REMIND students that Zero-based plans always have an accept number of 0 and reject on 1 defect.

Document findings in accordance with DCMA policy. Results of examinations are Government data used in adjusting the risk likelihood in the risk profile and





modifying the surveillance plan. Adjustments can include switching to tightened or reduced sampling when the data is accumulated.

40. EXPLAIN what happens after the QAS inspection.

Inform supplier of inspection results. The terminology (non-acceptance) used in specifications equates to “DCMA speak” of Accept/Reject.

Assure supplier complies with disposition in accordance with contractual requirements in the event of a rejected lot.

Document findings and issue CARs if nonconformities are found.

POINT OUT that a rejected lot may result in the request for a deviation permit, which is discussed later

BOTTOM LINE: Report findings to the supplier, whether or not there’s anything wrong. If the QAS is a non-resident, he/she lets the supplier know the results before leaving the facility. The QAS and supplier must work as a team for resolution, ensuring DCMA gets the best product for the customer.

41. EXPLAIN that FAR 52.246-2 requires suppliers to only tender an acceptable product. The supplier should be able to demonstrate that the product conforms.

Results of sampling can adjust the level of sampling performed based on the system being used. Sampling levels are reduced, normal and tightened.

TRANSITION into switching.

42. DISCUSS summary points on the screen.





43. DISCUSS summary points on the screen.

44. DIRECT students to Distance Learning course CMQ200 for more detailed training on Statistical Sampling.

45. POINT OUT CMQ200 topics include various Sampling Methods and application of Mil-Std-1916 and ANSI/ASQ Z1.4-2008 Sampling Systems.

46. ENTERTAIN questions at this point before moving to review questions.





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