What is quality sampling?
Quality sampling is the process of selecting a subset of a batch of products in order to assess the quality of the whole batch. Following an effectively managed sampling plan allows a business to reach conclusions about the quality of their products with a level of certainty and confidence. You may be interested in quality sampling because you want more control in ensuring the quality standard of products you receive from your suppliers, or as a quality control measure for the benefit of your customers and their satisfaction.
Why should my business conduct quality sampling?
Introducing an effective quality sampling system will have many different benefits for your business. A quality sampling system is essential for complying with regulations and standards across many industries, particularly in manufacturing. Conducting quality sampling on your products provides evidence that each batch has met your client’s specifications. Some customers will even require evidence of quality samples having passed testing for every batch they receive in order to accept that delivery – you can find out more about this below under should I use Acceptance Sampling?
Introducing a quality system is also a great marketing win – both your current and potential customers will be impressed at this improved commitment to the quality of your products.
Conducting quality sampling allows you to catch quality issues before shipping. This not only improves customer satisfaction, as mistakes are caught before involving your customer, but reduces the costs associated for your business if a product is rejected on delivery, such as redelivery charges and even fines.
Lastly, having a robust quality sampling system, as part of an overall culture of high standards at your business, reduces the likelihood of substandard products being created in the first place. This can save your business money and effort over time, and improve overall standards, not just those relating to product quality.
What do I need to consider to begin quality sampling?
It would be great if all businesses could run quality checks on every product they produce – for some manufacturers, that can even be the expectation. For example, if you produce luxury watches, it would make sense to conduct quality checks on every unit.
However, for most businesses, this would be too costly to implement, and also a heavy logistical burden. For some businesses, assessing quality would require products to be damaged or left unsaleable, so checking every unit would not only be ill-advised, but impossible.
As a business owner or quality manager, you need to decide on a sample size for each product which will give you and your customers confidence in its quality, and meet your regulatory needs, without overly effecting costs.
You need to find a balance between the time and effort your sampling schedule requires and the return you get from each product. A product which is sold in higher volumes or at a higher price point can justify a larger or more comprehensive sampling routine.
You also need to consider any regulatory or customer obligations with regard to your sampling.
What are the limitations when deciding a sample size?
What are my stakeholder requirements for quality sampling?
When deciding to implement a quality sampling system, it is important to consider the needs of your stakeholders, not just your business.
Your industry might have specific procedures or common practise when it comes to quality sampling. You may be obligated to undertake sampling in a way which has been specified by regulations or standards you are required or desire to meet. For example, the FDA specifies Binomial Staged Sampling Plans which must be followed to obtain or maintain compliance.
You also need to consider that sampling routines may be required for occupational safety, such as air sampling, safety sampling, or noise-level sampling. It may be more cost effective and efficient to include all forms of sampling, not just those for product quality, when implementing a sampling plan.
What type of sampling plan should I use?
You may think taking a random sample of the batch each time is an effective way of conducting quality sampling. However, there are issues with this approach. If the sample was not representative of the batch, inferior quality units will still be shipped to your customer’s, and with no agreement regarding this risk in place, they will feel dissatisfied, and you may both be left negotiating. This issue is exacerbated when time is a key factor, such as with perishable items. You are also at risk of sampling errors.
With a more sophisticated approach, such as using AQL inspections, your customers have a clear idea of the risk they are taking on, and the acceptable range of defects, and what action should be taken, is agreed prior to production. This means that both you and your customers feel satisfied and understand what is required. When using AQL you agree with your customer what the AQL (Acceptance Quality Limit) and level of inspection for each batch of product should be, for example, AQL =15% means that the accepted percentage of units with defects in a batch is 15 percent.
With AQL inspections, to follow good practise, you can decide to set AQL (Acceptance Quality Limit) to slightly higher than your customer would. In some industries, such as pharmaceutical, it may make sense to set these limits to zero – that is, to only accept a batch where no defects were found at all. You can find out how to calculate these limits, including by using ISO 2859-1 tables, here. Depending on the needs of your customers and your industry, it may also be appropriate to classify defects as critical, major and minor.
How do I set up a quality sampling system?
How do I continuously improve our quality sampling system?
As with any aspect of quality control you implement, your quality sampling system will only remain effective it is continuously reviewed and improved.
Ensure that adding new products to your quality sampling plan is part of your product development procedures and that agreements about sampling are made when you make new agreements with your suppliers and customers.
Make sure quality sampling is considered when you review your non-conformances and your audits. Sometimes when trying to resolve an issue, increasing sampling temporarily can help maintain customer satisfaction whilst the root cause is being determined.
Finally, make sure you are speaking to staff at all stages of production to see how they think sampling could be improved – often this is where best ideas will come from!