Funny I was having a very similar conversation in a class I was teaching yesterday. First AIAG’s ‘vested interest’ is only in making things easy for the auto industry. They have dumbed things down to simply make them easier for the Supplier quality engineers and managers so they can check the box. They have no real interest in making things ‘better’. ASQ has done the same thing with many of the methods they support in their certification exams. Thinking is hard. Putting numbers into a spreadsheet is easy. Certainly many of the ‘popular’ methods came about in the 40s and 50s before calculators and computers and EXCEL. So we got simple little ditties, because they were easy not because they were correct. If you read the articles of many of ASQ’s founders they were against this simplification.
My night school statistics professor told us that the sample standard deviation chart contains more information than the range chart, and I realized that the reason for the R chart was that it is much easier to subtract the smallest measurement from the largest than to calculate the standard deviation with a slide rule (and hand calculations for sums and differences also are required). People still use the R chart by habit today. I suspect that the median rather than x-bar chart was used for the same reason.
I think AIAG wants its methods to work, though, and has put a lot of effort into developing methodology for MSA and
FMEA. I don't really like MSA for attributes regardless of whose method is used unless, for example, it involves a go/no go gage that is set to a specific dimension, in which case the equipment variation can actually be estimated. The AIAG manual, however, includes a correction factor for a specific sample size and I don't know how it was derived. I was however able to get the formulas for derivation of the d*2 factors so I am more comfortable with those.
I also investigated some of the math in ANSI/ASQ Z1.9, and some of the calculations involve the noncentral t distribution (which Minitab and StatGraphics will handle)--I am always happier when I understand how something works.