Quality is the absence of variation. Any variation from the optimum represents lost quality. Minimum and maximum specifications do not define quality. They merely describe how much loss is acceptable before we can no longer ship the product.

Yes, I hear you. Of course all processes will vary. Of course we will never totally drive out variation. Nor will we ever truly have a quality product. That doesn’t make it any less worthy a goal. So for our purposes here, a quality product is one that is perfect, with no variation. A good product is one which has acceptable variation and a bad product is one which varies outside the min/max specifications.

Now that we all agree on what quality is, how do we get as close as possible to it?

Up to the early 70’s or so, most manufacturing emphasized quality control (QC). There sometimes seem like an infinite array of ideas about what QC is but it basically comes down to this: Catch bad products before they go to the customer or the next production step. Typically this relies on either human or automated inspection and rejection of “bad” products. Inspection may be 100% as in the case of a vision system. Alternately QC may sample some statistically significant portion of the products with acceptance/rejection based on bad products in the sample.

One possibility, sometimes overlooked, is false positives. These are good products that are rejected in error. This is often caused by overly sensitive inspection. In general, it is better to reject a good product than to accept a bad one, so false positives will be a natural tendency of QC.

After rejection, there is the problem of either fixing the defect or discarding the product, both of which have additional costs.

Quality control adds no value to the product. The only thing it does, when successful, is prevent further losses. When the inspection system misses a bad product, it doesn’t even do that.

In the beginning of the 70s and really taking hold in the 80s and 90s, manufacturers started addressing quality assurance (QA) along with quality control. QA has the same ultimate goal as QC, preventing bad product to the customer but with a different focus. QA does not try to catch bad products. Rather, QA emphasizes designing and operating the process so that it can’t make bad products in the first place. Some manufacturers even go so far as to eliminate in-process inspection and cut back on final inspection.

There are 2 ideas behind eliminating the inspection. First, if the process truly is incapable of making a bad product, there will be nothing for the inspectors to catch. Second, if there is inspection, it dilutes the commitment to making a quality product. Subconsciously the attitude can be “Well, if the process messes up, the inspectors will catch it.” It sounds a bit scary to me, like the tightrope walker working without a net. I do understand and applaud the idea behind it though.

So now when you hear the terms Quality Assurance and Quality Control, you will know that they are not synonyms. One, QC, focuses on inspecting bad products out. The other focuses on building quality in.

QA, doing it right the first time, seems like the better approach to me.