Transcript Document

IES 331 Quality Control
Chapter 15
Acceptance Sampling by Variables
Week 14
September 8-13, 2005
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Dr. Karndee Prichanont
IES331 1/2005
Acceptance Sampling by variables
Advantages vs Disadvantages
ADVANTAGES
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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

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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
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defective items
Dr. Karndee Prichanont
IES331 1/2005
Type of Sampling Plans for Variables
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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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Dr. Karndee Prichanont
IES331 1/2005
Basics of Variable Sampling Plan
In case of one side specification
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Dr. Karndee Prichanont
IES331 1/2005
Caution in the use of Variable Sampling
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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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Dr. Karndee Prichanont
IES331 1/2005
Designing
Variable
Sampling Plan
with a Specified
OC Curve
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Dr. Karndee Prichanont
IES331 1/2005
Illustrated Example using Nomograph
The 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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Dr. Karndee Prichanont
IES331 1/2005
Illustrated
Example using
Nomograph
p1 = 0.02,
p2 = 0.10,
alpha 0.10, and
beta 0.05.
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Dr. Karndee Prichanont
IES331 1/2005
Military Standard: MTL STD 414
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MIL STD 414 is a lot-by-lot acceptance-sampling plan for
variables
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Focal point is the AQL which ranges from 0.04% to 15%
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Five general inspection levels
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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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Dr. Karndee Prichanont
IES331 1/2005
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
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