Sampling plans are usually selected for given quality levels such as AQL,

, LQL, and

. The selected plan will have to be repeatedly used for each of the submitted lots if the quality levels remain unchanged. If the quality history is good, as evidenced by consecutive lot acceptances, then it may be desirable to reduce the amount of inspection. If the quality history is found to be bad (more lot rejections), then it is essential to tighten the inspection either by increasing the sample size or reducing the acceptance numbers. A specific plan selected for given quality levels has no mechanism to adjust according to the observed quality history. In such situations, the use of more than one plan is desirable with an associated rule for using each plan.
An acceptance sampling scheme is a specific set of procedures which usually consists of acceptance sampling plans in which lot sizes, sample sizes, and acceptance criteria, or the amount of 100% inspection, are related. Such schemes typically contain rules for switching from one plan to another. MIL-STD-105 E (1989) is an example for a sampling scheme.
A normal inspection plan is the plan used when the process is considered to be operating at, or slightly better than, the AQL.
A tightened inspection plan is a plan for using stricter acceptance criteria than those used in normal inspection .
A reduced inspection plan is a plan which permits smaller sample sizes than those used in normal inspection .
The main objective of using tightened inspection is to exert pressure on the producer when the quality is poorer than the AQL by introducing a higher rate of rejection.
When the level of the submitted quality is sufficiently good, reduced inspection offers sampling economy. For example, consider the following single sampling plans having three different inspection intensities.
Normal: n = 125, Ac = 1, Re = 2
Tightened: n = 200, Ac = 1, Re = 2
Reduced: n = 50, Ac = 0, Re = 1
These plans can be operated in a sampling scheme provided that there are rules for switching between them.
MIL-STD-105 E (1989) is an AQL indexed standard. Here, the AQL is defined as the percent defective (or the maximum number of defects per 100 units) that for the purposes of sampling inspection, is considered satisfactory as a process average .
MIL-STD-105 E contains three inspection intensities namely, normal, reduced and tightened plans. Single, double and multiple sampling plans are available, and plans of same inspection level are matched. This means that one may use single, double or multiple sampling plans depending on the sampling economy or reduction in complexity required, giving the same level of protection to the producer or consumer.
In order to implement the switching rules, a table of limit numbers for reduced

inspection is also available. The scheme also contains supplementary tables and graphs of limiting quality values, OC curves, ASN curves for double and multiple sampling plans which are not contained in this Study Guide. The essential tables are
Lot size - sample size code letters.
Master tables for single sampling - normal, reduced and tightened (3 tables).
Master tables for double sampling - normal, reduced and tightened (3 tables).
Master tables for 7-stage multiple sampling - normal, reduced and tightened (3 tables).
Limit numbers for reduced inspection.
For reduced plans (the final stage ones for double and multiple plans), Re does not equal Ac+1. A result between Ac and Re signals a switch to normal inspection. For example, consider the reduced inspection single sampling plan for Code K, AQL=1%, namely n = 50, Ac = 1 and Re = 4. While on reduced inspection, if two defectives are found the lot will be accepted but a switch to normal inspection will take place.
The following steps are recommended for obtaining a sampling scheme from MIL-
STD-105 E tables. Decide on the AQL.
1.
Decide on the type of sampling plan (single or double or 7-stage multiple).
2.
Decide on lot size.
3.
Obtain the sample size code letter for Level II of General Inspection Level (or any other designated level) for given lot size.
4.
Refer to the appropriate master tables to obtain the required normal, tightened and reduced plans using the AQL and the sample size code letter found in Step
4.
5.
Obtain the limit numbers for reduced inspection using the AQL and the number of sample units for the last 10 lots.
6.
Use Appendix A4.4 to get the switching rules.
7.
Obtain the other relevant performance measures such as LQ, ASN, AOQ and
ATI curves (not available in the Study Guide)
Example :
To obtain the MIL-STD-105 E scheme for the desired AQL of 0.65% and lot size
2000, one proceeds as follows:
Step 1: AQL = 0.65.
Step 2: single sampling.
Step 3: lot size = 2000.

Step 4: sample size code letter = K .
Step 5: Master tables yield: normal inspection plan n = 125 Ac = 2 Re = 3. tightened inspection plan n = 125 Ac = 1 Re = 2. reduced inspection plan n = 50 Ac = 1 Re = 3.
Step 6: Limit number for reduced inspection = 4 for AQL = 0.65 and the number of sample units for 10 lots = 1250.
Steps 7 & 8: Consult MIL-STD-105 E or ANSI/ASQC Z1.4 standard tables.
The scheme starts with normal inspection for the first lot and the switching rules shall apply from then on. If one fails to apply the switching rules and uses only normal plans, protection to the consumer will be lost. Consulting supplementary tables and graphs of the standard may also be necessary in order to have some idea of the ASN,
AOQL etc.
MIL-STD-105 E contains OC curves for individual sampling plans only. The OC curve of the scheme, ie the steady state or composite OC curve after implementing the switching rules, is not available in the standard.
The civilian version of the MIL-STD-105 E is the international standard: ISO
2859:1985-91 (also BS (British Standards Institution): 6001:1989-91), Sampling
Procedures for Inspection by Attributes which corresponds to MIL-STD-105 E. The
ISO standard 2859:1985-91 includes the composite OC curves and modifies the rule for discontinuation of inspection to '5 lots not being accepted while on tightened inspection'.