Upload
pinnuu
View
226
Download
0
Embed Size (px)
Citation preview
8/8/2019 Amit Om Presentation
http://slidepdf.com/reader/full/amit-om-presentation 1/8
8/8/2019 Amit Om Presentation
http://slidepdf.com/reader/full/amit-om-presentation 2/8
8/8/2019 Amit Om Presentation
http://slidepdf.com/reader/full/amit-om-presentation 3/8
8/8/2019 Amit Om Presentation
http://slidepdf.com/reader/full/amit-om-presentation 4/8
6. Process or Product Monitoring and Control
6.2. Test Product for Acceptability: Lot Acceptance Sampling
6.2.1. What is Acceptance Sampling?Contributions of
Dodge and Romig to acceptance
sampling
Acceptance sampling is an important field of statistical quality
control that was popularized by Dodge and Romig and originallyapplied by the U.S. military to the testing of bullets during World
War II. If every bullet was tested in advance, no bullets would beleft to ship. If, on the other hand, none were tested, malfunctions
might occur in the field of battle, with potentially disastrous
results. Definintion of Lot
AcceptanceSampling
Dodge reasoned that a sample should be picked at random from
the lot, and on the basis of information that was yielded by the
sample, a decision should be made regarding the disposition of the lot. In general, the decision is either to accept or reject the lot.
This process is called Lot Acceptance Sampling or just
Acceptance Sampling ."Attributes" (i.e.,
defect counting)will be assumed
Acceptance sampling is "the middle of the road" approach between no inspection and 100% inspection. There are two major
classifications of acceptance plans: by attributes ("go, no-go")
and by variables. The attribute case is the most common for acceptance sampling, and will be assumed for the rest of this
section. Important point A point to remember is that the main purpose of acceptance
8/8/2019 Amit Om Presentation
http://slidepdf.com/reader/full/amit-om-presentation 5/8
sampling is to decide whether or not the lot is likely to be
acceptable, not to estimate the quality of the lot.Scenarios leading
to acceptance
sampling
Acceptance sampling is employed when one or several of thefollowing hold:
•
Testing is destructive• The cost of 100% inspection is very high
• 100% inspection takes too long
A lot acceptance sampling plan (LASP) is asampling scheme and a set of rules for
making decisions. The decision, based oncounting the number of defectives in a
sample, can be to accept the lot, reject the
lot, or even, for multiple or sequentialsampling schemes, to take another sample
and then repeat the decision process.Types of acceptance plans to choose from LASPs fall into the following
categories:
• Single sampling plans:. Onesample of items is selected at
random from a lot and thedisposition of the lot is
determined from the resulting
information. These plans areusually denoted as (n,c) plans
for a sample size n, where the
lot is rejected if there are more
than c defectives. These are the
most common (and easiest)
plans to use although not themost efficient in terms of
average number of samples
needed.
• Double sampling plans: After the first sample is tested, there
are three possibilities:
1. Accept the lot
8/8/2019 Amit Om Presentation
http://slidepdf.com/reader/full/amit-om-presentation 6/8
2. Reject the lot
3. No decision
If the outcome is (3), and asecond sample is taken, the
procedure is to combine theresults of both samples and
make a final decision based onthat information.
• Multiple sampling plans: This
is an extension of the double
sampling plans where morethan two samples are needed to
reach a conclusion. The
advantage of multiple sampling
is smaller sample sizes.• Sequential sampling plans: .
This is the ultimate extension of multiple sampling where items
are selected from a lot one at a
time and after inspection of each item a decision is made to
accept or reject the lot or select
another unit.
• Skip lot sampling plans:. Skip
lot sampling means that only afraction of the submitted lots
are inspected. Definitions of basic Acceptance Sampling
termsDeriving a plan, within one of thecategories listed above, is discussed in
the pages that follow. All derivations
depend on the properties you want the plan to have. These are described using
the following terms:
• Acceptable Quality Level
(AQL): The AQL is a percentdefective that is the base line
requirement for the quality of
the producer's product. The producer would like to design a
sampling plan such that there is
a high probability of accepting
a lot that has a defect level less
8/8/2019 Amit Om Presentation
http://slidepdf.com/reader/full/amit-om-presentation 7/8
than or equal to the AQL.
• Lot Tolerance Percent
Defective (LTPD): The LTPDis a designated high defect level
that would be unacceptable tothe consumer. The consumer
would like the sampling plan tohave a low probability of
accepting a lot with a defect
level as high as the LTPD.
• Type I Error (Producer's
Risk): This is the probability,
for a given (n,c) sampling plan,of rejecting a lot that has a
defect level equal to the AQL.
The producer suffers when thisoccurs, because a lot withacceptable quality was rejected.
The symbol is commonly
used for the Type I error andtypical values for range from
0.2 to 0.01.
• Type II Error (Consumer's
Risk): This is the probability,
for a given (n,c) sampling plan,
of accepting a lot with a defect
level equal to the LTPD. Theconsumer suffers when this
occurs, because a lot with
unacceptable quality was
accepted. The symbol is
commonly used for the Type II
error and typical values rangefrom 0.2 to 0.01.
• Operating Characteristic (OC)
Curve: This curve plots the
probability of accepting the lot
(Y-axis) versus the lot fractionor percent defectives (X-axis).The OC curve is the primary
tool for displaying and
investigating the properties of a LASP .
• Average Outgoing Quality
(AOQ): A common procedure,
8/8/2019 Amit Om Presentation
http://slidepdf.com/reader/full/amit-om-presentation 8/8
when sampling and testing is
non-destructive, is to 100%
inspect rejected lots and replaceall defectives with good units.
In this case, all rejected lots are
made perfect and the onlydefects left are those in lots that
were accepted. AOQ's refer to
the long term defect level for this combined LASP and 100%
inspection of rejected lots
process. If all lots come in with
a defect level of exactly p, andthe OC curve for the chosen
(n,c) LASP indicates a
probability pa of accepting such
a lot, over the long run the AOQ can easily be shown to be:
where N is the lot size.