Sunday, November 9, 2014

Reasons to test

I tend to be a packrat for interesting articles.  I'll come across something interesting, open the page, and then move on.  Consequently, I'll pile up 20, 30 or more open links on Chrome for iPad until I finally break down and binge read.

Today is a lazy Sunday morning, so I binged.  One of those stored links dated to way back in July about reasons to test.  It's a nice little summary of four canonical test types.  However, it missed at least two test reasons that are specific to volume manufacturing: binning and SPC.

Suppose your company makes thousands of widgets that have variations in a key parameter.  You therefore have two choices: spend time and money reducing that variation to acceptable limits, or find customers that desire those variations.  If you opt for the second one you spend time and money to implement the testing for that parameter, and you use the test results to put the parts into different categories for different customers.  That's binning.

I'm not going to try to describe statistical process control (SPC).  I've taken a couple of small courses in in it, and I've applied it to manufacturing testing at a couple of different companies.  But I'm no expert.  Go here for a good summary, and follow the external references for more detail.  All I will say is:

  1. SPC is a requirement for high volume manufacturing.  
  2. You need lots of data for SPC.
  3. You have to test to get that data.

So why did the article's author leave out these items?  I can think of three reasons.  First, maybe he thought they fell under the verification or validation categories.  I could maybe buy the verification for SPC argument, but that's as far as I would go.  Second, his testing experience is in low-volume industries (i.e. - certain military markets) where SPC or binning isn't useful.  Third, he just wrote the article quickly to meet a deadline without thinking it through.

That last reason is a little harsh.  But this lazy Sunday morning is also kind of cold and overcast, so I'm a little morose.

(the internets love cats)

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