Transcript ANSI/ASQC Z1.9

Chapter 15
Other Acceptance Sampling
Techniques
Introduction to Statistical Quality Control,
4th Edition
15-1. Acceptance Sampling by
Variables
15-1.1 Advantages and Disadvantages of
Variables Sampling
Advantages
•
•
•
Smaller sample sizes are required
Measurement data usually provide more information
about the manufacturing process
When AQLs are very small, the sample sizes required by
attributes sampling plans are very large.
Introduction to Statistical Quality Control,
4th Edition
15-1. Acceptance Sampling by
Variables
15-1.1 Advantages and Disadvantages of Variables
Sampling
Disadvantages
•
The distribution of the quality characteristic must be
known
•
A separate sampling plan must be employed for each
quality characteristic that is being inspected.
•
It is possible that the use of a variables sampling plan
will lead to rejection of a lot even though the actual
sample inspected does not contain any defective items.
Introduction to Statistical Quality Control,
4th Edition
15-1. Acceptance Sampling by
Variables
15-1.2 Types of Sampling Plans Available
• Two types of variables sampling procedures
1. Plans that control the lot or process fraction
defective (or nonconforming). [Procedure 1]
2. Plans that control a lot or process parameter
(usually the mean). [Procedure 2]
Introduction to Statistical Quality Control,
4th Edition
15-1. Acceptance Sampling by
Variables
15-1.3 Caution in the Use of Variables Sampling
•
•
•
The distribution of the quality characteristic must be
known in order to use variables sampling
The usual assumption is that the parameter of interest
follows the normal distribution. This is a critical
assumption.
If the normality assumption is not satisfied, then
estimates of the fraction defective based on the sample
mean and standard deviation will not be the same as if
the parameter were normally distributed.
Introduction to Statistical Quality Control,
4th Edition
15-1. Acceptance Sampling by
Variables
15-1.3 Caution in the Use of Variables Sampling
•
•
It is possible to use variables sampling plans
when the parameter of interest does not have a
normal distribution.
If the form of the distribution is known, it is
possible to devise a procedure for applying a
variables sampling plan.
Introduction to Statistical Quality Control,
4th Edition
15-3. MIL STD 414
(ANSI/ASQC Z1.9)
15-3.1 General Description of the Standard
•
MIL STD 414 is a lot-by-lot acceptance-sampling plan
for variables introduced in 1957.
•
Sample size code letters are used as in MIL STD 105E,
but the same code letter does not imply the same sample
size in both standards.
•
Sample sizes are a function of the lot size and the
inspection level.
•
All sampling plans assume the quality characteristic of
interest is normally distributed.
Introduction to Statistical Quality Control,
4th Edition
15-3. MIL STD 414
(ANSI/ASQC Z1.9)
15-3.1 General Description of the Standard
•
MIL STD 414 is divided into four sections:
–
–
–
–
A: general description of the sampling plans including
definitions, sample size code letters, and OC curves for the
plans.
B: variables sampling plans based on the sample standard
deviation for the case in which the process or lot
variability is unknown.
C: variables sampling plans based on the sample range
method
D: variables sampling plans for the case where the process
standard deviation is known.
Introduction to Statistical Quality Control,
4th Edition
15-3. MIL STD 414
(ANSI/ASQC Z1.9)
15-3.3 Discussion of MIL STD 414 and ANSI/ASQC Z1.9
ANSI/ASQC Z1.9 is the civilian counterpart of MIL STD 414.
Differences and revisions
1. Lot size ranges were adjusted to correspond to MIL STD
105D
2. Code letters assigned to the various lot size ranges were
arranged to make protection equal to that of MIL STD 105E
3. AQLs of 0.04, 0.065, and 15 were deleted
4. Original inspection levels I, II, III, IV, and V were relabeled
S3, S4, I, II, III, respectively.
5. Original switching rules were replaced by thos eof MIL STD
105E, with slight revisions.
Introduction to Statistical Quality Control,
4th Edition