vdoc

156 Part III: Inspection and Test
individuals using, maintaining, or depending on the product, or a
defect that judgment and experience indicate is likely to prevent
performance of the unit. In practice, critical characteristics are
commonly inspected to an AQL level of 0.40 to 0.65 percent if not 100
percent inspected. One hundred percent inspection is recommended
for critical characteristics, if possible. Acceptance numbers are always
zero for critical defects.
• Major defect. A major defect is a defect, other than critical, that is likely
to result in failure or to reduce materially the usability of the unit of
product for its intended purpose. In practice, AQL levels for major
defects are generally about one percent.
• Minor defect. A minor defect is a defect that is not likely to reduce
materially the usability of the unit of product for its intended purpose.
In practice, AQL levels for minor defects generally range from 1.5
percent to 2.5 percent.
Levels of Inspection
Part III.B
There are seven levels of inspection used in ANSI/ASQ Z1.4-2003: reduced inspection,
normal inspection, tightened inspection, and four levels of special inspection.
The special inspection levels should only be used when small sample sizes are
necessary and large risks can be tolerated. When using ANSI/ASQ Z1.4-2003, a set
of switching rules must be followed as to the use of reduced, normal, and tightened
inspection.
Switching Procedures. Switching rules are graphically shown in Figure 15.3.
Types of Sampling
ANSI/ASQ Z1.4-2003 allows for three types of sampling:
1. Single sampling
2. Double sampling
3. Multiple sampling
The choice of the type of plan depends on many variables. Single sampling is the
easiest to administer and perform, but usually results in the largest average total
inspection.
Double sampling under ANSI/ASQ Z1.4-2003 results in a lower average
total inspection than single sampling, but requires more decisions to be made,
such as:
• Accepting the lot after the first sample
• Rejecting the lot after the first sample
• Taking a second sample
• Accepting the lot after the second sample
• Rejecting the lot after the second sample