Sampling plans are used to determine how we will choose the batch to be checked. Usually, an inspector only checks a part of the whole. A portion should be enough in order to determine the quality of the items to be delivered.
Which sampling plan do you need for your production?
This is a crucial part since it can dictate the outcome of your inspection. Choosing the inappropriate plan may lead to inaccurate results. For us to answer that question, it is best to take a look first at the different types of sampling plans to know which one fits your inspection.
1. Single-stage sampling plan
This plan is mainly about having a definite number of items (n) to be checked. The number n relies on the size of the whole and also the inspection level. If the number of defective items is less than the AQL limit, it succeeded in passing the test.
2. Double-stage sampling plan
In this type of sampling plan, a smaller quantity (n1) will be taken. If the number of defects in n1 is more than the agreed limit, more samples will be chosen. It is a double verification.
3. Sampling plan by attributes
When we say sampling plan by attributes, there can only be two possible results. It can only be “non-defective” or “defective”. There is no in-between.
A sample of n units is randomly selected from a lot of N units. If there are C or fewer defectives, the batch is accepted. If there are more than C, it will be rejected.
Let’s say that you have a batch of 10,000 earphones. You will check 90 pieces of them. If there are up to 2 defective earphones, then you may accept it. If there are more than 2 defectives, then the entire batch of earphones should be rejected.
4. Sampling plan by variables
Unlike sampling plan by attributes, sampling plan by variables gives way for a more complex evaluation. Things such as the length of the product and exact findings are taken into account in decision making.
If you need to check the computer’s height and computer’s width, you need two separate sampling plans. In this sampling plan, we assume that the distribution of the quality characteristic is normal. Though this is a more complex process than sampling plan by attributes, an advantage of using this is that you only need a smaller number of sample size compared to the other.
5. Rectifying sample plan
The perfect situation to use this sample plan is when defects that are found can be fixed immediately. It has similarities with sampling plan by attributes but it considers that the batch is of higher quality after the inspection. If there is a case of inspection failure, the whole batch of items should be inspected.
6. Continuous sampling pan
This is usually used when products are made piece by piece in a continuous pattern. This sample plan is made up of several stages.
The first stage is when each piece is checked. This is called the “screening”. After a certain quantity were found to be in a satisfactory state, only certain pieces are checked randomly. This stage is called the “sampling”. But when a piece is found to be defective, the process will go back to the screening stage.
This plan can be best used if you want to make sure that 100% of the items that you are checking are in a good condition. This may take more time than other sampling plans but it assures that all are working properly.
7. Accept on Zero sampling plan
This is also known as zero-defective plan. This sampling plan is seldom used in China mainly because only batches with no defective unit will be accepted. They dictate the acceptance of the batch only if no non-conforming items are detected. Because of this sample plan, producers are forced to deliver high quality products. Making this a favorable sampling plan for consumers. An advantage of this plan is that only a small number of samples needs to be checked.
How Asia Quality Control can help you?
We offer a rigorous and complete training to the inspectors we recruit by providing them quality control related courses and examinations. We also ensure that they pass the tests so that they are able to perform inspections and understand the different points to verify.
Our trained quality inspectors identifies and records the non-compliance report, and look for performance improvements.