The sampling criteria:
- The main sampling criteria are lot sizes, inspection levels, acceptable quality levels, sample size code letters, and acceptance and rejection points.
- Three types of sampling plans, single, double, and multiple.
- Three types of Inspection levels: Normal, reduced, and Tightened inspections.
- The AQL and the sample size code letter shall be used to obtain the sampling plan.
The selection of sample size is explained below:
Step 1: Decide the lot size and the inspection level.
Step 2: Assume a lot size of 151- 280 and inspection level G- II (general inspection II), then the inspection level code is obtained as “G” from the below table 1. In short, the lot size and the inspection level allow us to select the sample size code letter.
Step 3: Select the type of sampling plan: single/ double /multiple sampling. The acceptability of the lot is based on the percent of nonconforming inspection.
Step 4: Select the Inspection level- Normal/ reduced / Tightened inspection.
Step 5: The AQL and the sample size code letter shall be used to obtain the sampling plan.
Step 6: If consider AQL as 4, and the code for a lot size of 151- 280 is G (table1) and single sampling plan with normal inspection level. Then the required sample size will be 32 units with the Acceptance number and Rejection numbers are 3 and 4 respectively (Table 2.A).
*The tables are from ISO 2859-1
The international standards for sampling plans are:
- ISO 2859-1: Sampling procedures for inspection by attributes – Part 1: Sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection.
- ANSI/ ASQ Z1.4 & Z1.9: Sampling Procedures and Tables for Inspection by Attributes.