1 IES 331 Quality Control Chapter 15 Acceptance Sampling by Variables Week 14 September 8-13, 2005.

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IES 331 Quality Control

Chapter 15Acceptance Sampling by Variables

Week 14September 8-13, 2005

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Dr. Karndee Prichanont IES331 1/2005

Acceptance Sampling by variables Advantages vs Disadvantages

ADVANTAGES

The same OC curve can be obtained with a smaller sample size than would be required by an attributes sampling plan

Measurement data usually provide more information about manufacturing process than attributes data

When acceptable quality level as are very small, sample sizes required by attributes sampling plans are very large

DISADVANTAGES

Distribution of OC curve must be known

Most standard plans assume distribution of quality characteristic is normal

A separate sampling plan must be employed for each quality characteristic that is being inspected

Possible to reject a lot even though the actual sample inspected does not contain any defective items

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Type of Sampling Plans for Variables Type 1: Plans to control the lot or process

fraction defective (nonconforming)

Type 2: Plans to control a lot or process parameter (process mean)

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Basics of Variable Sampling PlanIn case of one side specification

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Caution in the use of Variable Sampling

Usual assumption is that the parameter of interest follows the normal distribution

If parameter of interest is not normally distributed, estimates of the fraction defective will not be the same as if normally distributed

Difference between estimated fraction defectives may be large when dealing with very small fractions defective

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Designing Variable Sampling Plan with a Specified OC Curve

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Illustrated Example using NomographThe density of a plastic part used in a cellular phone is required

to be at least 0.70 g/cm3. The part supplied in large lots. It is desired to have p1 = 0.02, p2 = 0.10, alpha 0.10, and beta 0.05.

Assume that the variability is unknown but will be estimated by the sample standard deviation

a) Find the appropriate variable sampling plan

b) Suppose that a sample of the appropriate size was taken, and sample average is 0.73, and sample standard deviation is 1.05 x 10-2

. Should the lot be accepted or rejected?

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Illustrated Example using Nomograph

p1 = 0.02,

p2 = 0.10,

alpha 0.10, and

beta 0.05.

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Military Standard: MTL STD 414 MIL STD 414 is a lot-by-lot acceptance-sampling plan for

variables

Focal point is the AQL which ranges from 0.04% to 15%

Five general inspection levels ___________________________________

Sample sizes are a function of the lot size and the inspection level

Provision is made for normal, tightened, and reduced inspection

Quality characteristic of interest is assumed to be normally distributed

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Illustrated Example using MTL STD 414

An inspector for a military agency desires a variable sampling plan for use with an AQL of 1.5%, assuming that lots are of size 7000. If the standard deviation of the lot or process is unknown, derive a sampling plan using MIL STD 414

1 IES 331 Quality Control Chapter 15 Acceptance Sampling by Variables Week 14 September 8-13, 2005.

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