Product inspection and sampling inspection standards

The concept of product inspection

• Unit product: A basic unit divided for the purpose of inspection, such as a truck, 1 meter of cotton cloth, 1 kilogram of cement, a pair of shoes, etc.

• Inspection lot: A group of unit products requiring inspection. Abbreviated as a batch. The unit products comprising an inspection batch should not differ in any essential way, subject only to random fluctuations. Therefore, an inspection batch should consist of unit products of the same type and grade, produced under essentially the same production conditions and within approximately the same period of time.

• batch: The number of units in a batch of products that require inspection.

• Fatal flaws: Defects that may create hazards or unsafe conditions for the use and maintenance of the product or for people involved therewith; or defects that impair the important and ultimate basic functions of the product.

• Serious flaws: A defect that is different from a fatal defect but can cause failure or significantly reduce the expected performance of the product.

• Minor defects: Defects that do not significantly reduce the expected performance of the product, or defects that deviate from the standard but only slightly affect the effective use or operation of the product.

Product inspection classification

Product inspection can be divided into full inspection, sampling inspection, purchase inspection, intermediate inspection, finished product inspection, factory inspection, inventory inspection, supervision inspection, counting inspection, metrological inspection, destructive inspection, non-destructive inspection, etc.

Application scenarios of sampling inspection

Sampling inspection is often used in the following situations:
a) the inspection is destructive;
b) During the inspection, the object being inspected is a continuum;
c) a large number of products;
d) Many inspection items;
e) hope to reduce the inspection cost;
f) As an inspection for production process control.

Random Sampling Method:

• Simple random sampling: Random number table method, dice rolling method.

• Periodic systematic sampling: A method of sampling at regular intervals.

• Stratified sampling: From an inspection batch that can be divided into different sub-batches (or layers), samples are drawn from different layers in a specified proportion.

Sampling inspection standards and their systems

1. Sampling inspection standards for counting and measurement

• The attribute sampling inspection standard uses the results of attribute sampling inspection as the indicator for determining quality characteristics. The national standards that have been formulated are:
  a) GB2828-1987 "Batch-by-Batch Inspection Sampling Procedure and Sampling Table (Applicable to Inspection of Continuous Batches)"
  b) GB2829-1987 "Periodic Inspection Attribute Sampling Procedure and Sampling Table (Applicable to Inspection of Production Process Stability)"
  c) GB8051-1987 "Procedures and tables for sequential sampling inspection by attributes"
  d) GB8052-1987 "Single-level and multi-level continuous sampling inspection procedures and tables"

• The measurement sampling inspection standard uses the results of attribute sampling inspection as the indicator for determining quality characteristics. The national standards that have been formulated are:
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  a) GB6378-1986 "Measurement Sampling Inspection Procedure and Table for Defective Product Rate"
  b) GB8053-1987 "Standard Single Sampling Inspection Procedure and Table for Defective Product Rate"
  c) GB/T8054-1995 "Metric Standard Single Sampling Inspection Procedure and Table for Average Values"

2. Adjustment and non-adjustment sampling inspection standards

Adjustable sampling inspection standards, such as GB2828-1987 and GB6378-1986, can adjust the strictness of the sampling inspection plan according to the quality changes of consecutive batches, so as to guide manufacturers to pay attention to product quality or reduce sampling costs while ensuring batch quality.

3. Single sampling, double sampling, multiple sampling and sequential sampling inspection standards

Use the inspection procedures of GB2828-1987

a) Specify the quality characteristics of unit products <br>In product technical standards or order contracts, the quality characteristics of unit products such as technical performance, technical indicators, and appearance must be clearly specified.
b) Classification of unqualified products <br>According to actual needs, unqualified products are divided into three categories: A, B and C. If necessary or necessary, there can be more than three categories.
c) Qualified quality level <br>In product technical standards or order contracts, the qualified quality level should be determined by negotiation between the ordering party and the supplier. The qualified quality level is the average upper limit of the process of continuously submitted inspection batches that is considered acceptable.
d) Inspection Level Specifications <br>The standard specifies general inspection levels: I, II, III, and four special inspection levels: S-1, S-2, S-3, and S-4. Unless otherwise specified, inspection level II is generally adopted. When lower discrimination is required, general inspection level I may be specified. When higher discrimination is required, general inspection level III may be specified. Special inspection levels are only applicable when smaller samples must be used and a greater risk of misjudgment can or must be tolerated.
e) Formation and submission of inspection batches <br>Unit products are simply collected to form inspection batches.
f) Inspection Strictness <br>Inspection strictness refers to the degree of strictness of the inspection received by the submitted batch. This standard stipulates three different levels of inspection strictness: normal inspection, strict inspection and relaxed inspection.
Unless otherwise specified, at the beginning of the inspection, a normal inspection should be used.
Check the strictness transfer rules as shown in the attached table.
g) Selection of Sampling Plan Type <br>The standard provides three types of sampling plans: single, double, and five-fold. The decision to use a single, double, or five-fold sampling plan is usually made by comparing the management costs and average sample sizes of the corresponding sampling plans of different types.
h) Retrieval sampling plan <br>Use the table retrieval sampling plan given in the standard according to the sample size code and qualified quality level
i) Sampling <br>Sampling should be done in a way that is representative of the quality of the batch. When the inspection batch consists of several strata, stratified sampling is used to extract samples.
When using the two-fold and five-fold sampling plans, each sample should be drawn from the entire batch.
The time for taking samples can be during the batch formation process or after the batch is formed.
j) Sample inspection
k) Determination of qualified or unqualified batch by batch inspection
l) Disposal after batch inspection

Appendix: Strictness Transfer Rules
Conversion direction and conversion conditions

• Normal inspection → Strict inspection: If 2 batches out of no more than 5 batches fail the initial inspection (excluding the batches submitted for inspection again), the next batch of inspection will be transferred to the stricter inspection.

• Strict inspection → normal inspection: When conducting a tightened inspection, if five consecutive batches pass the initial inspection (excluding batches submitted for inspection again), the next batch of inspection will be transferred to normal inspection.

• Normal inspection → Relaxed inspection: When conducting a normal inspection, if the following conditions are met, the next batch of inspections will be transferred to a relaxed inspection:

  1. 10 consecutive batches (excluding those submitted for inspection again) have passed normal inspection
     2) The total number of unqualified products (or defects) in samples taken from 10 consecutive batches or more than 20 consecutive batches is less than or equal to the limit number listed in Table 1 of the standard

  2. Normal production

  3. The competent quality department agreed to switch to relaxed inspection

• Relaxed inspection → Normal inspection: When conducting a relaxed inspection, if any of the following situations occurs, the next batch of inspections will be transferred to normal inspections:

  1. A batch of relaxed inspections failed

  2. Abnormal production

  3. The responsible quality department deems it necessary to return to normal inspection

• Stricter inspection → Suspend inspection: After the tightened inspection begins, when the number of unqualified batches (excluding batches submitted for inspection again) accumulates to 5 batches (excluding the number of unqualified batches that appeared in the previous tightened inspection), the inspection according to this standard will be suspended.

Products that are exempt from inspection may also be considered for random inspections.