Many leads who want to do a quality control check or inspection, asked me how many pieces will be inspected and checked, out of XXX amount, or can we check all of the quantity, and other similar questions. In this article, you’ll find the answer of those questions.
Do we check them all?
No…
Why don’t we check them all?
We use the AQL ANSI/ASQ Z1.4- 2003 basis. The AQL of a sampling plan is a level of quality routinely accepted by the sampling plan. It is generally defined as the percent defective – Non Conformances- (defectives per hundred units X 100%) that the sampling plan will accept 95% of the time. This means lots at or better than the AQL are accepted at least 95% of the time and rejected at most 5% of the time. Associated with the AQL is a confidence statement one can make. If the lot passes the sampling plan, one can state with 95% confidence that the quality level of the lot is equal to or better than the AQL (i.e., the defective rate of the lot < AQL). On the other hand, if the lot fails the sampling plan, one can state with 95% confidence that the quality level of the lot is worse than the AQL.
So, you don’t have to worry 🙂 It’s a generally acceptable procedure.
Will all checkpoints be checked on all samples?
Inspectors do have actual checkpoints to follow, but… Not all checkpoints will be checked on all samples. Sometimes, it depends on the products.
Why?
For some products (mostly related to machine), it’s impossible to check all the functions in 1 man/day. Checking just 1 function could take the entire 1 man/day! But, don’t be misled by that statement… Here are some examples to narrow the idea:
1. Checking the carton’s size
It usually makes sense to measure one carton, and to have a quick look at the others (“are they roughly the same size?”)2. Abusing a product for 5 minutes to see if it breaks or gets deformed easily
Doing just this test on all samples will take 1,000 minutes, or 17 hours.
So, in conclusion,
1. Things we can check quickly (in the case of your product: visual defects, fitness test between lid and body, etc.) is checked on the whole sample size.
2. Things we can’t check quickly is divided in two types:
– What is very important to customer (critical to quality), doesn’t take a very long time to check or might vary substantially from one sample to the next due to the production process. Actually, the whole sample can be checked, but it will take more man/days and it all depends on the customer. Dear customer, your request is our command 🙂. When problem is classified as a defect, just the way a visual defect is counted. It makes sense since all inspection samples are checked at this point (and the statistics from ISO 2859-1 were respected).
– Other checkpoints will be checked on a smaller sample size. But, let’s say a point is checked on 5 pieces and one problem is found on 1 of the 5 samples. The inspector can simply consider this problem as critical, or in another words, the checkpoint is failed. At this point, buyer will decide whether it is critical or not.