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  • Roger Yao

Inspection sampling scheme for fitness equipment: How many samples should be taken during inspect?


Summary:


For fitness equipment products with complex structures, how many samples should be selected during final inspection for quality checking and testing? This is the question customers have to answer when they make an inspection plan.


This article briefly describes the classification of international inspection sampling standards and the calculation method of sampling quantity, then explains the reasons why general products adopt sampling level II while fitness equipment products need to adopt smaller sampling levels. Combined with years of quality management for fitness equipment and working experience in third-party inspection companies, the author recommends FQC sampling scheme for fitness equipment products, and gives suggestions of "dynamic adjustment of sampling levels" in long-term quality inspection management.


Content List:



The content of this article can also be used as a reference when buyers and suppliers discuss a new inspection sampling scheme.



1. The international standard for quality inspection sampling plan



Customers, who buy from China’s factories, often conduct pre-shipment quality inspections to verify whether the finished products meet the expected quality requirements. Random sampling is the first action when quality inspector or the designated third-party inspection agency conduct inspection, means, randomly pick up a certain number of samples from the completed bulk goods for subsequent inspection.


Then, the question is: What kind of sampling standard do inspectors follow?


Many countries and organizations have their own quality inspection sampling standards, such as


Although these standards come from different countries and organizations, they are synergistic and follow the same principles. These standards cover seven optional sampling levels, namely: I, II, III and S1, S2, S3 and S4.


Different sampling levels reflect the difference in the number of samples selected. Hence, the final number of sample size depends on two factors: bulk order quantity and the predetermined sampling level.


* ISO2859: Sampling standard ( in red box are sampling level-II and corresponding sample size)


For example: If the customer order quantity is 1000 pieces of clothing, then the sample size should be J, that is, 80 pieces of clothing. According to this, the inspector should randomly select 80 samples from 1000 pieces of finished products.


QC knowledge FQA:


Q1: Can sampling level be expressed with percentage (%) ?


Actually, the sampling levels mentioned in the international standards are not equivalent to the percentage rate (%) that is commonly understood. Some people refer to this sampling method as AQL sampling method although this expression is not very rigorous.


Q2: So why don’t we just set a sampling percentage % for samples pick-up?


Because it involves the combination of the number of samples and the AQL judgement rule, and for the topic of AQL ( Acceptable Quality Limit) , we will discuss it in another article writing.



2. Commonly-used sampling level for general consumer products


There are thousands of consumer product categories in the international trading business, and the vast majority of them are with relatively simple functions and structures, such as clothing, shoes and hats, toys, stationery, decorations, and electronic products.


sampling level 2 for general products
Consumer products with simple structure

Customers tend to hire third-party inspection companies to conduct pre-shipment quality inspection when they buy from other countries or unfamiliar suppliers. When determining the sampling level, customers, especially the retail buyers, choose a sampling level that is applicable to most product categories of their purchase, it is likely to be sampling level II.


And if there is no explicit sampling level requested by customer, the third-party inspection agency will set level II as default. Therefore, the sampling level II has become more and more popular among many customers. Objectively speaking, the sampling level II is suitable for most products with simple process, structure and function, but NOT for products with complex configuration and structure.



3. Why sampling level II is not the best choice for fitness equipment?


The problem comes when some fitness equipment customers apply the same sampling level scheme that normally are used on simple consumer products to fitness equipment.


Quality check for bench, cross trainer, treadmills
Fitness equipment with complex structure

There have been complaints from some fitness equipment suppliers in China, saying that the third-party inspection company designated by the customer adopts the sampling level II , which takes up a lot of factory manpower, material resources, space and equipment. By doing with that, inspectors spend more time on unpacking samples, checking basic information, appearance issues, and taking pictures rather than on critical quality points such as function, performance, internal structure, etc., suppliers stressed that such way of inspection makes little sense and just wasting of time and money.


There are also dissatisfaction from customer who hired third-party inspection company because they still received many quality complaint from the after-sales department even though many samples were checked and a PASS inspection result was given during inspection.


So, for products such as treadmills, exercise bikes, and multi-gym station, what critical quality points should inspectors prioritize?


We know that for large and complex fitness equipment products, consumers are more concerned about product safety, functionality and operational parameters; The appearance issues are also important but should be put behind the issues of safety, functionality, and operational parameters.


internal check for exercise bike
Internal structure check and component check

These critical quality problems are often originated from improper product design, specification configuration, mold fixtures setting. Therefore, the occurrence of corresponding problems are mostly in bulk. Professional inspectors can defect the problems and the proportion of defects in bulk goods by a deep check and investigation on just a few samples instead of many samples.


For example, according to the EN957 standard, there are unsafe extrusion points between product components, which is a safety defect. Such problems are caused by unreasonable structural design. If inspector finds one sample with such defects, it is likely all the rest products are with the same problem.


For another example on treadmill, there is a discrepancy between the actual speed and marked speed, which is a problem of running parameters. This kind of phenomenon is most likely caused by the wrong configuration of the transmission system. Instead of repeatedly inspecting dozens of samples, the inspector should spend time to find the cause to know which part of configuration was wrong.


To sum up, in order to ensure an effective quality inspection, the inspectors do not necessarily have to check many samples, but should check more deeply and carefully with their professional know-how and understanding in fitness equipment and production processes.



4. FQC recommended sampling scheme for inspection


Then, what is the reasonable sampling size during quality inspection for fitness equipment?


To ensure an effective and efficient quality inspection, FQC adopts a practical and reasonable inspection plan sampling scheme for large fitness equipment :


sample size for fitness equipment QC
FQC sampling plan for fitness equipment

  • Downgrade the total sample size from the level II to level I

  • Classify inspection items into 3 categories: general inspection items, specific inspection items and special inspection items

  • Each inspection classification adopts own sampling level

However, customers and suppliers should work together to set the classification of different inspection item, and the following are the general classification principles from FQC:

  • General inspection Items: product information, packaging, appearance, components, specification check, etc.

  • Specific inspection items: assembly, use function, performance(part), part safety test, etc.

  • Special inspection items: internal structure, durability, performance(part) and destructive testing, etc.



For example, if the order quantity of treadmills is 300 units, according to the sampling level recommended by FQC, a total of 20 units should be selected, and the general inspection should be performed firstly; then, among the 20 units, 8 units will be randomly selected for assembly, functional parameters, safety test; meanwhile, 3 units are randomly selected for special inspection items such as internal structure and durability.



5. What is dynamic inspection sampling scheme?


Even though FQC has its own inspection sampling scheme, we do not recommend customers applying the same sampling level as always. It should be dynamically adjusted according to real situation over time, which is called “ dynamic sampling scheme”. The basic principle of the dynamic sampling scheme is to increase or decrease the sampling level according to the actual changes of product phases, supplier credibility and quality performance.




When making dynamic adjustments to sampling levels, it is recommended to consider the following:



Different product phases: for newly developed products, given the instability of the product, the sampling level II may be adopted; and after certain time, when the product becomes mature, sampling level l could be adopted.


Reliability of supplier management system: After audited by customers or FQC, if the supplier has strict control in the production process and a well-maintained quality management system, downgrading sampling level can be considered and adjusted.


Historical quality performance: If the supplier quality performance in the past is excellent, the sampling level downgrading could be considered; if the past quality performance is unstable and the quality complaints are serious, then the sampling size should be increased, or even introduce inline inspection during mass production.


6. Conclusion


When we do quality inspection on fitness equipment products, the most issue is not about checking more samples or checking less samples, it is about how we check. Given the complexity of fitness equipment, we have to optimize the limited inspection time and focus on the critical quality aspects while still covering general inspection items. In order to achieve the effectiveness and economy of quality management, customers and suppliers should have an open discussion to find out a reasonable and practical inspection sampling scheme.



Author: Roger Yao (cs01@fitqs.com)


If you have any questions in terms of quality inspection or set inspection sampling scheme, please contact our quality expert for help!








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