Tag: sampling

The sampling app you’ve always wanted… ANSI Z1.4 2003 made easy

ansi_samplingPro QC’s team of technical engineers has developed an Android and iOS application that takes the guesswork out of the accept or reject determination of an on-site quality inspection depending on customizable major and minor AQLs (Acceptable Quality Limits). This application has been incredibly useful for our team on-site, in addition to many of our clients. For convenience, we also have a web-based application.

More on defect classifications here.

To learn more about ANSI Z1.4 2003 sampling, a previous article discusses the topic in more detail.

Which AQL do you choose?

One of the most frequent questions we receive regarding on-site quality inspections relates to selecting AQLs, or Acceptable Quality Levels.  AQLs represent the maximum percent defective that you consider acceptable.

Pro QC’s Assistant Operations Manager in China, Cesar Marsical, address the question below:

The selection of AQLs and sampling plan for a given lot size depends on too many factors to permit the issuance of a “pre-selected” standard set of plans for specified lots. Each user should select AQLs and sampling plans that are tailored to best meet their needs.  Pro QC can assist with this process.

Some of the factors that must be considered prior to selecting the AQLs are:

1) Classifications or categories of defects such as Critical, Major and Minor – Critical defects would generally require zero defects, which means the highest AQL value should be imposed.  Major defects would generally require a lower AQL than those for minor defects.

2) Process capabilities under good commercial practice with respect to the defects in question – For example, if under normal production process, the defect levels cannot be kept below 2.0 percent defective, the selection of an AQL of 0.15 percent defective, although desirable for the defects in question, may not be practical.

3) Consumer preferences – These may require higher AQL’s or permit lower AQL’s than process capabilities would indicate.

4) Time and cost required to sample and inspect a lot under various AQLs – The smaller the AQL, the more time and cost of inspection.

Some of the factors that may be considered prior to selecting the sampling plans for a given lot size includes:

1) The applicable AQLs – The AQL dictates, among other things, the smallest sample size that can be used and the size of the “jumps” from one sample size to the next larger one.

2) The relative ability of the plans to discriminate between “good” and “bad” lots. – Although several plans in these standards have the same AQL, they differ in their ability to reject lots worse than the AQL’s. The OC (Operating Characteristics) curve in the standards of this subpart provides the basis for determining the discriminating ability of each plan.

3) The amount, time, and cost of sampling required.

4) The size and value of the lots relative to the producer and consumer protection a sampling plan affords.  One may be willing to take larger risks of passing “bad” lots that are small or of lesser value than they would for larger more valuable lots.

5) The knowledge about the lot(s) to be submitted for inspection – Lots consisting of product produced under essentially the same conditions may require smaller sample size than those consisting of product produced by different shifts and different raw stocks for example.

6) The record of the quality level of previously submitted lots – The sample size can be smaller for lots submitted from a supplier with a consistent record of quality levels significantly better than the specified AQL(s) than sample sizes for the supplier whose records shows considerable variability in quality, “borderline” supplies or products worse than the AQL.

Based on the above, decide what quality levels you are willing to live with and what quality level that you are not willing to live with.

Contact us for more info, or assistance with this process.

 

Acceptance sampling isn’t scary

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.

Some advice…

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.