How Inspection Sampling Plan Works in Clothing Industry

What is Inspection Sampling Plan?
Inspection sampling plan is a chart that helps the inspector to control the quality of garment, he probably inspect the garments only a portion of the whole lot. Buyer mainly decided how many sample size he pick and what sampling plan does he follow?

Inspection Sampling Plan is a chart devised by American scientists to enable to conduct inspection of merchandise by random sampling given in this chart.

Inspection Sampling Plan in the Apparel Industry
Fig: Inspection Sampling Plan in the Apparel Industry

An inspection sampling plan in the clothing industry involves the systematic selection of a subset of items from a larger batch or production run to assess the quality of the entire batch. This process is crucial for quality control to ensure that products meet the specified standards and requirements. There are various methods for inspection sampling, but one commonly used approach is the Acceptance Sampling Plan.

Here are the key components of an inspection sampling plan in the clothing industry:

1. Sampling Method:

  • Random Sampling: Items are selected randomly from the production batch. This ensures that each item has an equal chance of being chosen, providing a representative sample.
  • Stratified Sampling: The production batch is divided into subgroups (strata) based on certain characteristics (e.g., size, color). Samples are then randomly selected from each stratum.

2. Sample Size:

  • The number of items selected for inspection is a critical factor. It is determined based on statistical considerations, such as the acceptable quality level (AQL) and the lot size. The AQL specifies the maximum percentage of defective items that can be accepted.

3. Acceptance Criteria:

  • Criteria for determining whether the batch is accepted or rejected are established in advance. This includes the maximum allowable number of defects or deviations from the quality standards.

4. AQL (Acceptable Quality Level):

  • AQL is a pre-established standard for the maximum acceptable defect rate. It is expressed as a percentage of the total production lot. The sampling plan is designed to ensure that the actual defect rate does not exceed the AQL with a certain level of confidence.

5. Inspection Levels:

  • Inspection levels (e.g., I, II, III) indicate the degree of scrutiny applied during the inspection process. Level I involves the least sampling, while Level III involves the most intensive inspection.

6. Sampling Plan Tables:

  • These tables provide guidelines for determining the sample size and acceptance/rejection criteria based on the lot size and AQL. The most commonly used standard for this purpose is the ANSI/ASQ Z1.4-2008 standard.

7. Sampling at Different Stages:

  • Sampling may occur at different stages of production, including raw materials, in-process production, and finished goods. This helps identify and address quality issues at various points in the manufacturing process.

8. Documentation:

  • Detailed records of the sampling plan, inspection results, and any corrective actions taken should be maintained. This documentation is essential for quality control audits and continuous improvement efforts.

Implementing an effective inspection sampling plan helps manufacturers identify and rectify quality issues early in the production process, reducing the likelihood of delivering defective products to customers.

Inspection Sampling Plan have 5 Parts:

  1. Lot size
  2. Sample size
  3. Acceptable quality level (AQL)
  4. Accept number and
  5. Reject

1. Lot size:
Lot size represents full consignment of goods from which samples are drawn.

2. Sample size:
Sample size is the number of goods drawn randomly for inspection.

3. Acceptable quality level (AQL):
Acceptance Quality Level (AQL) refers to the maximum number of defective items that could be considered accepted during the random sampling of and inspection.
During inspection defects are found into 3 categories:

  1. Critical: Must be 100% accurate. There is no range.
  2. Major: Normally 2.5%
  3. Minor: Normally 4%

4. Accept number:
Accept number is quantity rejected from the sample but indicates that, if the numbers of rejects are within limit, the full lot is accepted.

5. Reject number:
If the numbers of rejects are without limit, then the full lot is rejected.

Why Quality Inspection Sampling Plan is Essential for Apparel Industry?
Buyers always try to ensure that goods delivered to them by the manufacturer must complete with the specified quality standard. The name of the plan is means of which quality standard of goods is ensured is called quality acceptance sampling plan or quality inspection sampling plan.

In this system, buyer defines quality standard of goods. Buyer worried only about those major defects which may impair functional quality of goods and thus their sale ability. Renowned buyers generally supply the seller with a list of such major defects and inform of the Inspection Sampling Plan that must be pursued.

Correctness of statistical acceptance sampling plan or statistical auditing largely depends upon weather size and color-wise sample drawing was representative of the whole lot or not.

Sampling plan is selected in such a way that quality is ensured within a statistical confidences limit of 95% or above. In garments industry, the following Acceptable Quality Levels (AQL) are used 1.5%, 2.5%, 4%, 6.5% etc. AQL means allowable percent defectives in a sample.

American military standard ABC-STD-105 defines that” The AQL is the maximum percent defective (or the maximum number of defects per hundred units) that, for purpose of sampling inspection, can be considered satisfactory as a process average.

Practically quality acceptance sampling plan & AQL started finding its use since World War-II. Large number of tables and methods has been devised to put sampling plan &AQL to proper and correct use.

Classification or Types of Sampling Plan:
A. Based on how many times samples are randomly drawn from a lot Inspection Sampling plan is classified into 3 categories:

  1. Normal or Single Sampling Plan
  2. Double Sampling Plan
  3. Multiple Sampling Plan

For example, These categories of sampling plan are mention in below with a table.

Types of inspection sampling plan based on number of sampling

Sl. Sampling type Sample size Cumulative Sample size Acceptance No. Rejection No.
1. Single Sampling Plan 50 50 1 2
2. Double Sampling Plan
1st Sampling
2nd Sampling
3. Multiple Sampling Plan
1st Sampling
2nd Sampling
3rd Sampling

B. Based on thoroughness or toughness of sampling, inspection sampling plans are classified in the following manner:

  1. General or Normal Sampling
  2. Relaxed or Reduced Sampling
  3. Tightened Sampling

In case, the vendor is new for the buyer, tightened sampling is used for the first two consignments at least, until confidence is created between buyer and seller. If buyer is fully satisfied with the first two consignments, normal sampling inspection may be applied thereafter. In case of general quality goods or when relationship between buyer and seller is very good relaxed sampling may find application.

For example, These categories of sampling plan are mention in below with a table.

Sl. Sampling type Lot size Sample size Acceptance No. Rejection No.
1. General or Normal Sampling 50 13 3 4
2. Relaxed or Reduced Sampling 50 13 1 2
3. Tightened Sampling 50 5 0 1

Different Methods of Inspection Sampling Plans:
In the clothing industry, inspection sampling plans are crucial for maintaining quality standards and ensuring that finished products meet customer expectations. The methods used for inspection sampling in the clothing industry are often adapted from general quality control practices. Here are some commonly employed methods:

1. AQL Sampling (Acceptable Quality Level):
AQL sampling is one of the most widely used methods in the clothing industry. It involves setting an acceptable quality level (the maximum percentage of defective items allowed) and determining sample sizes based on this level. The ANSI/ASQ Z1.4-2008 standard provides tables for AQL sampling plans.

2. Random Sampling:
Random sampling involves selecting items randomly from a production batch. This method helps ensure that each item has an equal chance of being chosen for inspection, providing a representative sample of the entire lot.

3. Stratified Sampling:
Stratified sampling involves dividing a production batch into subgroups (strata) based on specific criteria, such as size, color, or production line. Samples are then randomly selected from each stratum. This method is useful when variations in quality are expected within different subgroups.

4. In-Process Sampling:
Inspection is not limited to the final product. In-process sampling involves checking the quality of items at various stages of production. This allows for the identification and correction of issues before the final product is completed.

5. Finished Goods Sampling:
Sampling can focus specifically on finished goods. This involves selecting a random sample of finished products from the production batch and inspecting them for defects or deviations from quality standards.

6. Attribute Sampling:
Attribute sampling involves classifying items as either conforming or non-conforming based on specific attributes, such as stitching quality, color accuracy, or fabric integrity. This method is particularly applicable to the clothing industry where specific characteristics define product quality.

7. Variables Sampling:
Variables sampling measures continuous characteristics, such as dimensions, weight, or tensile strength. This method provides a more detailed assessment of the quality of specific attributes.

8. 100% Inspection:
In some cases, particularly for high-value or critical items, 100% inspection may be employed. This means every item in the production batch is inspected for quality. While thorough, this method can be time-consuming and may not be practical for large production runs.

9. Lot-by-Lot Inspection:
This method involves inspecting each lot or shipment separately. The acceptance or rejection of a particular lot is based on the results of the inspection. This method is suitable for situations where each production run is distinct.

10. Sampling Based on Critical Points:
Identify critical points in the apparel production process that significantly impact quality. Focus sampling efforts on these critical points to ensure that potential issues are caught early in the manufacturing process.

The choice of a specific method depends on factors such as the type of clothing, production volume, critical quality characteristics, and industry standards. Manufacturers often create detailed quality control plans that specify the sampling method, sample size, acceptance criteria, and inspection frequency. Regular adherence to these plans helps maintain consistent product quality.

You may also read:

  1. In-line Quality Inspection Process in Apparel Industry
  2. Final Inspection Procedure in Garment Industry
  3. Classification of Defects Zone in Apparel During Visual Inspection

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