Acceptance Sampling
Prepared by
Anthony Lizza
Penn State University
What will be covered?
• What is acceptance sampling?
• How/When would you use it in
your organization?
• Acceptance sampling explained.
• How acceptance sampling
works.
• An exercise.
• Summary
What is acceptance
sampling?
• Acceptance Sampling
– Statistical quality control
technique, where a random sample
is taken from a lot, and upon the
results of the sample taken the lot
will either be rejected or accepted.
What is acceptance
sampling?
• Accept Lot
–
Ready for customers
–
Not suitable for customers
–
Sample and determine if in
acceptable limits
• Reject Lot
• Statistical Process
Control(SPC)
What is acceptance
sampling?
• Purposes
– Determine the quality level of an
incoming shipment or, at the end
production
– Ensure that the quality level is
within the level that has been
predetermined
What is acceptance
sampling?
• Can be either 100% inspection,
or a few items of a lot.
• Complete inspection
– Inspecting each item produced to
see if each item meets the level
desired
– Used when defective items would
be very detrimental in some way
Acceptance Sampling
method
ACCEPTANCE SAMPLING
TAKE SAMPLE
INSPECT SAMPLE
Type title here
DECISION
ACCEPT LOT
REJECT LOT
SAMPLE AGAIN
DECISION
RETURN LOT TO SUPPLIER
100% INSPECTION
What is acceptance
sampling?
• Problems with 100% inspection
– Very expensive
– When product must be destroyed
to test
– Inspection must be very tedious so
defective items do not slip through
inspection
How/When would you
use it?
• Acceptance sampling
advantages
–
–
–
–
Less handling damages
Fewer inspectors to put on payroll
100% inspection costs are to high
100% testing would take to long
How/When would you
use it?
• Acceptance sampling
disadvantages
– Risk included in chance of bad lot
“acceptance” and good lot
“rejection”
– Sample taken provides less
information than 100% inspection
How/When would you
use it?
• Between your organization and
outside world
• Samples taken run through
“filter,” either passing or
rejecting it
– Also filter from suppliers to you
How/When you would
use it?
• When products in use could be
damaged easily
• When using new suppliers
• When new products produced
• When current supplier in
question
• Testing whole lot could be
harmful
How/When you would
use it?
• Determine how many units, n, to
sample from an lot
• Determine maximum number of
defective items, c, that can be
found before the lot is rejected
Acceptance sampling
explained
• Acceptable Quality levels(AQL)
– Number of defect percentage
allowed in a lot which can still be
considered accepted(Type I error)
• Lot Tolerance Percent
Defective(LTPD)
– Amount of defects that will come
with a lot of goods(Type II error)
Acceptance sampling
explained
• Sampling Plan
– Forms after n and c values have
been found
• Producers risk
– Risk associated with a lot of
acceptable quality rejected
Acceptance sampling
explained
• Consumers risk
– Receive shipment, assume good
quality, actually bad quality
• Alpha
– Type I error(producers risk)
• Beta
– Type II error(consumers risk)
Acceptance sampling
explained
•N
– Sample size taken for your
sampling plan
•C
– Where rejections would occur
when defects exceeded this
percent
Acceptance sampling
explained
• Operating characteristics
curve(OC)
– A graph, displaying standards at
which shipments would be
accepted
• First
– Determine AQL, a, LTPD, b,
– Take LTPD/AQL, this gives you the
n(AQL)
Acceptance sampling
explained
• Reference n(AQL) in the table
– C will be given as well when referencing
the table
• Next
– N(AQL/AQL)=n-sample size
– C= reject if more percent defects more
• The equations can be tricky but the
exercise later will help
How acceptance
sampling works
• Two classifications of
acceptance plans
– Attributes(“go no-go”)
– Variables
How acceptance
sampling works
• Attributes(“go no-go”)
– Defectives-product acceptability
across range
– Defects-number of defects per unit
• Variable(continuous)
– Usually measured by mean and
standard deviation
How acceptance
sampling works
• Remember
– You are not measuring the quality
of the lot, but, you are to sentence
the lot to either reject or accept it
An Exercise
• Determine the
– AQL, a, LTPD, B?
– Reference table providedc
LTPD/
AQL
nAQL
c
LTPD/
AQL
nAQL
0
44.890
.052
5
3.549
2.613
1
10.946
.0355
6
3.206
3.286
2
6.509
.0818
7
2.957
3.981
3
4.890
1.366
8
2.768
4.695
4
4.057
1.970
9
2.618
5.426
An Exercise cont.
• Assume a manufacturer purchases
wire from an outside vendor. The
wire vendor has an accepted quality
level of 1% and accepts a 5% risk of
rejecting lots below this level. The
manufacturer considers lots with 3%
defectives to be unacceptable and
assumes a 10% risk of accepting a
defective lot.
Develop a sample plan for the
manufacturer to be followed by the
inspection personnel
Summary
• Acceptance sampling is used by
organizations to determine if
there process’s are running
within a controlled limit and to
see if they should reject or
accept lots
Summary
• There are many basic terms you
need to know to be able to
understand acceptance sampling
– SPC, Accepts Lot, Reject Lot, Complete
Inspection, AQL, LTPD, Sampling Plans,
Producers Risk, Consumers Risk, Alpha,
Beta, Defect, Defectives, Attributes,
Variables.
Summary
• Advantages/Disadvantages of
acceptance sampling
• Purpose of acceptance sampling
• When to use acceptance sampling
• Equations involved
• Exercise
– Notes pages have many beneficial hints
and help better relate the material, the
answer to the exercise is also located on
a the notes page of the exercise.
Bibliography
• (Foster, S.Thomas: “Managing
Quality-An Integrative
Approach:pgs. 263-274:copyright
2001)
• “Quality Control in Furniture
Manufacturing”:Internet
http://www.mtc.com.my/publicati
ons/library/quality/qc40.htn
Bibliography
• “Process Analysis”:Internet
http://www.statsofinc.com/textb
ook/stprocan.html
• “Quality Control”: Internet
http://www.ms.ic.ac.uk/jeb/or/qc
ontrol.html