vdoc

158 Part III: Inspection and Test
Sample size
Plan
(percent)
Attribute 100
s unknown, range method 60
s unknown, s estimated from sample 40
s known 15
Part III.B
2. Variables sampling plans allow the determination of how close to
nominal or a specification limit the process is performing. Attribute
plans either accept or reject a lot; variables plans give information on
how well or poorly the process is performing.
Variables sampling plans, such as ANSI/ASQ Z1.9-2003, have some disadvantages
and limitations:
1. The assumption of normality of the population from which the samples
are being drawn.
2. Unlike attribute sampling plans, different characteristics on the same
parts will have different averages and dispersions, resulting in a
separate sampling plan for each characteristic.
3. Variables plans are more complex in administration.
4. Variables gauging is generally more expensive than attribute gauging.
ANSI/ASQ Z1.9-2003
The most common standard for variables sampling plans is ANSI/ASQ Z1.9-2003,
which has plans for: 1) variability known, 2) variability unknown—standard deviation
method, and 3) variability unknown—range method. Using these methods,
this sampling plan can be used to test for a single specification limit, a double (or
bilateral) specification limit, estimation of the process average, and estimation of
the dispersion of the parent population.
As in ANSI/ASQ Z1.4-2003, several AQL levels are used and specific switching
procedures for normal, reduced, and tightened inspection are followed. ANSI/
ASQ Z1.9-2003 allows for the use of the same AQL value for each specification
limit of double specification limit plans or the use of different AQL values for each
specification limit. The AQL values are designated ML for the lower specification
limit and MU for the upper specification limit.
Figure 15.4 summarizes the structure and organization of ANSI/ASQ
Z1.9-2003.
Appendix H provides some tables from the ANSI/ASQ Z1.9-2003 standard
that are used for the selection of variables sampling plans.
SEQUENTIAL SAMPLING PLANS
When tests are either destructive in nature or very costly, it may be advantageous
to use sequential sampling plans popularized by Wald (1973). These plans have
the advantage of greatly reduced sample sizes while providing good protection.