As it turns out, a significant number of people find the Pro QC site searching for sampling and sampling related terms such as ANSI Z1.4 or MIL-STD-105E (no longer in use). In speaking with organizations, I’ve also detected a certain level of fear regarding how to approach an acceptance sampling strategy.
Fear is unnecessary… In fact, sampling here is nothing more than a tool you’re going to use to provide a realistic (of course statistically relevant) assessment of quality and be able to ultimately determine whether or not product is meeting expectations without having to spend the time and money involved with a 100% inspection. This is a good thing.
Get your specs straight – Before you even start to think about AQLs and defect classifications, go back and make sure you’ve really evaluated your product and have created detailed product specifications. Once you have this information, go through each item and determine how you want to classify nonconformance (major, minor or critical). At Pro QC, we use the following general guideline for defect classifications:
- Critical – Any condition found which poses the possibility of causing injury or harm to, or otherwise endangering the life or safety of, the end user of the product or others in the immediate vicinity of its use.
- Major – Any condition found adversely affecting the product’s marketability and sale-ability or adversely affecting its required form, fit or function and which is likely to result in the end user returning it to the source from which is was purchased for refund or replacement.
- Minor – Any condition found which while possibly less than desirable to the end user of the product, does not adversely affect its marketability, sale-ability, form, fit or function and is unlikely to result in its return to the source from which is was purchased.
Special or General? – The ANSI Z1.4 tables show various sample sizes based on lot (batch) sizes in relation to Special and General levels. Here, you’ll want to go through your spec and determine what would require higher levels (general) of sampling or lower (special). For example, if you have an electronic product that requires functional testing (need to make sure it turns on maybe), special levels of testing makes sense because it may be considered destructive. The same goes for drop-testing to determine the packaging integrity. Time is an obvious important consideration, as it relates to not only cost but to shipment schedules as well. An example might look like this:
- Visual (cosmetic/workmanship) inspection – General Level 1
- Drop-testing (Package integrity ISTA testing) – Special Level 3
- Functional Testing – Special Level 1
Single or Double? – Pro QC uses ANSI Z1.4 single normal sampling as a standard, but we have had clients request double. If you look at the tables, you’ll note the difference is simply the number you look at. So, it goes back to time and cost, but you’re also considering your level of assurance with the supplier in general. You can look at less with double if the product meets expectations, but you’ll be taking a 2nd full sample set if not.
Know your AQLs – Acceptable Quality Limits are simply “the worst tolerable process average in percentage or ratio, that is still considered acceptable: that is, it is at an acceptable quality level.” To figure out what works best, play around with the numbers… Plug in your average lot sizes and levels and go through the row of scenarios. What are you comfortable with? An example might look like this:
- Major Defects 1.5
- Minor Defects, 4.0
- Critical Defects 0.10
Tweak it – Sampling strategy is something that can grow and evolve with your product. As you develop supplier relationships and analyze inspection feedback, you’ll be able to make adjustments as necessary.
Utilize the experts – Of course, a 3rd party quality assurance provider such as Pro QC, can provide you with expert advice and guidance or even evaluate your product and develop the specs/sampling for you. That both relieves apprehension and leverages expertise.
Note: International and national equivalents of the ANSI Z1.4 2003 standard exist as ISO 2859, NF06-022, BS 6001 and DIN 40080.