I. Composition of the complete product inspection specification
An effectively operating product inspection specification is the key basis for ensuring product quality, and it covers multiple important aspects.
First, there are requirements for quality characteristics. This is a detailed definition of various quality features of the product, such as the product's physical properties, chemical characteristics, appearance dimensions, etc. For different products, the requirements for quality characteristics can also vary greatly. Take electronic products as an example. People may focus on their electrical performance, stability, anti - interference ability, etc. For food, more attention is paid to its nutritional components, hygiene indicators, shelf life, etc. Defining the requirements for quality characteristics is like setting a precise quality "template" for the product, and subsequent inspection work will be carried out based on this benchmark.
The inspection (testing) method is a means to ensure accurate judgment of whether a product meets the requirements of quality characteristics. For different quality characteristics, corresponding inspection methods need to be adopted. For example, for the physical properties of a product, inspection can be carried out through methods such as measurement with measuring tools and mechanical tests; for chemical properties, it may be necessary to use chemical analysis instruments to detect the composition and content. Appropriate inspection methods can ensure the reliability and accuracy of inspection results, thus providing strong support for product quality evaluation.
Sampling rules are also an important part of product inspection specifications. In actual production, since the quantity of products is usually large, it is often unrealistic to inspect all products one by one. Therefore, sampling inspection is required. Sampling rules specify how to select samples from the overall products, including the quantity and method of sampling. Reasonable sampling rules can make the selected samples representative, so that the quality status of the overall products can be inferred from the inspection results of the samples.
Acceptance criteria are the standards for judging whether a product is qualified. It clarifies the circumstances under which a product can be accepted and those under which it needs to be rejected. For example, when the number of non - conforming products in the sample is lower than a specified value, the batch of products is considered qualified; otherwise, it is judged as unqualified. The formulation of acceptance criteria requires comprehensive consideration of factors such as the product's quality requirements, actual production conditions, and customer expectations.
In addition, sometimes the product inspection specification may also include the disposal after inspection and testing and the handling of non-conforming products. After non-conforming products are detected, there should be clear handling methods, such as rework, downgraded use, scrapping, etc. This can handle non-conforming products in a timely manner to prevent them from entering the market. At the same time, it also helps to analyze the causes of non-conformance, take corresponding improvement measures, and improve product quality.
If an enterprise has established an ISO9000 quality management system, the product inspection specifications can be implemented in accordance with the provisions corresponding to Article 8.3 of the system. The ISO9000 quality management system is a set of internationally recognized quality management standards. Its relevant provisions are scientific and authoritative, and can provide more standardized and systematic guidance for the enterprise's product inspection work.
II. Types of inspections for production - oriented enterprises
At present, there are mainly two types of inspections for production enterprises: 100% inspection and sampling inspection.
1. Full inspection
Full inspection involves inspecting all products one by one. The advantages of this inspection method are quite obvious. It can provide relatively accurate inspection results. Since each product is inspected, it can ensure that the product quality meets the requirements. However, full inspection also has obvious limitations. The inspection workload is extremely large, requiring a significant amount of manpower, material resources, and time. In some cases, such as when conducting destructive inspections, full inspection becomes unrealistic. For example, when inspecting the service life of light bulbs, once the test is carried out, the light bulbs will be damaged. If all light bulbs are tested in this way, all products will be destroyed, and the enterprise will be unable to sell products normally.
2. Sampling inspection
Sampling inspection involves drawing a sample (n) from the population (N), conducting a comprehensive inspection on the sample n, and then inferring the quality of the population based on the inspection results of the sample. Since only some products are inspected, there is a certain risk in sampling inspection. However, through scientific calculations and adjustments, this risk can be reduced to an acceptable level.
Sampling methods can be divided into two categories:
Percentage sampling method: This method involves selecting x% of the total population as a sample. However, the percentage sampling method is clearly unscientific. Taking a batch of products as an example, if the actual non - conforming product rate is 2%, and sampling is done at 10%. When the batch size N = 50, the sample size n = 5, the number of non - conforming products D = 50×2% = 1, and the acceptance probability Pa = 0.9; when N = 200, n = 20, D = 200×2% = 4, and the acceptance probability Pa = 0.654; when N = 500, n = 50, D = 500×2% = 10, and the acceptance probability Pa = 0.345. It can be seen that the larger the batch size, the lower the acceptance probability. This may lead some unscrupulous enterprises to "break the whole into parts", splitting large - batch products into small batches to get through the inspection.
Statistical sampling inspection: This method establishes a numerical model according to statistical laws and then calculates the sampling method, so it is relatively scientific. It can be further subdivided into attribute sampling inspection and variable sampling inspection.
Attribute sampling inspection: Make inferences about a lot based on the number of conforming products (or non - conforming products) in the sample, which is further divided into sampling by attributes (counting the number of non - conforming units) and sampling by defects (counting the number of non - conformities). Common relevant standards include GB/T2828.1—2003, GB/T13246—1991, etc. For example, when inspecting a lot of parts, determine whether the whole lot of parts is qualified by counting the number of conforming and non - conforming parts in the sample.
Measurement sampling inspection: Make inferences about the batch using the sample mean ($\overline{x}$) and the sample standard deviation (S). Although the measurement process is relatively complex, the sample size can be significantly reduced. For example, the GB/T6378 - 2002 standard. Taking a practical example, according to the code in GB/T2828.1, the corresponding sample size n = 50. If BG/T6378 is used instead, when the value of δ is unknown, n = 20; when δ is known, n ≤ 14. This shows that measurement sampling inspection can improve the inspection efficiency to a certain extent.
III. Key points of the commonly used technical sampling inspection standard GB/T 2828.1 for industrial enterprises in China
GB/T 2828.1 is a commonly used technical sampling inspection standard for industrial enterprises in China, which has specific scope of application and key points.
1. Scope of application
This standard is applicable to populations with large batch sizes. For products with large batch sizes, using this standard for sampling inspection can improve inspection efficiency and reduce inspection costs on the premise of ensuring a certain degree of inspection accuracy. At the same time, it requires that the quality information be continuous and adjustable, and it is not applicable to isolated batches. That is to say, this standard is more suitable for batches of continuously produced products, so that the sampling plan can be adjusted in a timely manner according to the changes in product quality.
2. Given conditions
To conduct sampling inspection using the GB/T 2828.1 standard, five known conditions need to be grasped in advance.
Lot size N: It refers to the quantity of the overall products. The size of the lot will affect the selection and determination of the sampling plan. Different lot sizes may correspond to different sample sizes and sampling rules.
Scheme types: Divided into single, double, and five - times sampling schemes. The single sampling scheme is relatively simple and convenient to operate. It only requires one sample draw to make a judgment. Although the double and five - times sampling schemes can reduce the number of sampled samples, the operation is slightly more complicated, requiring multiple samplings and judgments. Enterprises can select the appropriate scheme type according to their own production conditions and inspection requirements.
Inspection levels: There are four special inspection levels, namely S - 1, S - 2, S - 3, S - 4, and three normal inspection levels, namely Ⅰ, Ⅱ, Ⅲ. As the inspection level increases, the sample size gradually increases. Special inspection levels are applicable to situations where the requirement for inspection accuracy is not high or the inspection cost is relatively high; while normal inspection levels are applicable to general quality inspection requirements.
Acceptable Quality Limit (AQL): It is an important quality indicator, representing the upper limit (maximum value) of the average nonconforming product rate of continuously submitted batches. It is the quality standard for the submitted inspection batches in the sampling inspection with adjustment for attribute. The sampling inspection plan with adjustment for attribute can ensure that the buyer gets products with an average quality level corresponding to the AQL. AQL is expressed as a percentage (%) of nonconforming products. In GB/T 2828.1, if it is by piece counting, AQL ≤ 10; if it is by defect counting, it may reach up to 1000. For example, when the AQL is set at 2%, it means that the average nonconforming product rate of the submitted inspection batch cannot exceed 2%.
Strictness levels: There are three strictness levels: normal, tightened, and reduced. In Chapter Ⅸ of GB/T 2828.1, a set of transfer rules are stipulated, and there are corresponding sampling plan tables for reference. When the product quality is stable, normal inspection can be adopted; when the quality declines, tightened inspection is required to ensure the product quality; and when the quality remains stable and good for a long time, the inspection requirements can be appropriately relaxed.
3. Obtaining of the sampling plan
According to the above known conditions, the sampling plan can be obtained by looking up the table.
- Obtain the sample size code from Table 1 based on the lot size N and the inspection level IL. The sample size code is an important basis for subsequently determining the sampling plan.
- For a single sampling:
- During normal inspection, obtain the sample size n, the acceptance number Ac, and the rejection number Re from Table 2 - A according to the code letter and AQL.
- When using tightened inspection, find the corresponding values from Table 2 - B.
- When using reduced inspection, obtain the corresponding values from Table 2 - C.
- For double sampling:
- During normal inspection, based on the code letter and AQL, obtain the sample size n1 for the first sampling, the acceptance number A1 for the first sampling, the rejection number R1 for the first sampling, the sample size n2 for the second sampling, the acceptance number A2 for the second sampling, and the rejection number R2 for the second sampling from Table 3 - A.
- When conducting tightened inspection, find the corresponding values from Table 3 - B.
- When using reduced inspection, obtain the corresponding values from Table 3 - C.
- For the fifth sampling, and so on, obtain the sample size, acceptance number, and rejection number for each sampling from Tables 4 - A, B, and C according to the code letter and AQL.
4. Acceptance criteria
Sequential sampling: From a sample of n, the number of non-conforming products (d) is found. If d ≤ Ac, the batch of products is accepted; if d ≥ Re, the batch of products is not accepted. This is a simple and direct way of judgment, determining whether the products are qualified by comparing the number of non-conforming products with the acceptance number.
Double sampling: Detect the number of non-conforming products \(d_1\) from the sample of size \(n_1\). If \(d_1\leq A_1\), the lot of products is accepted; if \(d_1\geq R_1\), the lot of products is not accepted; if \(A_1\leq d_1\leq R_1\), a sample of size \(n_2\) needs to be drawn. Detect the number of non-conforming products \(d_2\) from the sample of size \(n_2\). If \(d_1 + d_2\lt A_2\), the lot of products is accepted; if \(d_1 + d_2\gt A_2\), the lot of products is not accepted. This sampling method requires two judgments and is more flexible, which can reduce unnecessary sampling to a certain extent.
Five - time sampling: By analogy, make a judgment based on the number of non - conforming products in each sampling and the corresponding acceptance numbers to determine whether the batch of products is qualified.