## Every answer to statistical problems lies within RSD

As I was teaching class the other day, I told the students I was going to reveal to them the one secret they needed to learn to understand every statistical test they would ever use. The secret was the one thing that would make statistics more of a reasonable science than a bunch of equations to memorize, the one thing they needed to pass my class. (OK, there is a lot more needed to pass the class, but without this one thing doing so is a lot harder.)

## You must balance risk and benefits when determining acceptability.

So I thought I was done with measurement system analysis after my last column, but I just finished reading Don Wheeler’s June 1 column, “Is the Part in Spec?” and the first thing I thought was, “Well, that was… complicated and ultimately unhelpful in answering the article’s title question.” I like a diversity of viewpoints, but they have to make sense. Does Wheeler’s? Let’s take a closer look.

## Can they co-exist?

We have recently covered a lot of ground on the topic of measurement system analysis (MSA). We talked about the basics of MSA, the potential study, the short-term study, and the long-term study. At this point you should have a pretty firm foundation in the importance and methods of good MSA studies for your research and production, as well as a practical tool to help you in doing measurement system analysis—the file "MSA Forms 3.22.xls" (gauge repeatability and reproducibility worksheets)— which is a free download from Six Sigma Online. In this article, I am going to tie up some loose ends and then talk about a frequent question, “Is MSA even possible with a destructive gauge?”

## Testing through time stability

Ahh, measurement system analysis—the basis for all our jobs because, as Lord Kelvin said, “… When you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind.” How interesting it is then, that we who thrive on data so frequently don't have any proof that the numbers we're using relate to the event we are measuring—hence my past few articles about the basics of measurement system analysis in “Letting You In on a Little Secret,” on how to do a potential study in “The Mystery Measurement Theatre, and on how to do a short-term study in “Performing a Short-Term MSA Study.” The only (and most important) topic remaining is how to perform a long-term study, which is the problem I left with you last month.

So read on to see how.

## Gauging your conformance decisions

n the past couple of articles, we have been having fun together testing whether a measurement device is usable for the crazy purpose of determining if we are actually making product in or out of specification. Last month, we performed a measurement systems analysis (MSA) “potential study” using a snazzy MSA spreadsheet (if I do say so myself)*. We found that the Hard-A-Tron was not only pretty highly variable (compared to our spec), but that the material we were measuring actually might have been changing over time. But a potential study was not enough for you, was it? You asked, nay demanded, that we perform a short-term MSA, and I, your humble servant, gave you the data to do so. After the jump, we will perform the analysis, so unless you are the type of person that flips to the back of the book to see if you want to read it, finish up your analysis, and then click to read more.

## Determining sources of variations

if you can’t trust your measurement system, you can’t do anything with the data it generates. Last month, in “ Letting You In On a Little Secret,” we talked about the purpose of measurement system analysis (MSA) and I gave you a neat spreadsheet that will do MSA for you, as well as some data (repeated after the jump) from the gauge you want to buy, the Hard-A-Tron. I also left you with a mysterious statement that this study was trickier than it appeared. This month I’ll start off answering a question I received, and then we will see how well the Hard-A-Tron did—and what mysterious thing was going on in the data. After that, if you are good, I’ll give you another set of data to further test a measurement device.

## 'Cause I’m a nice guy, I’m going to give you articles that explain the basics of MSA.

You know how sometimes you think everyone knows a secret that they haven’t let you in on? Well, I had the opposite happen to me the other day. I assumed everyone knew the purpose for measurement system analysis (MSA), a.k.a. gauge repeatability and reproducibility; but I found out that a number of people have a completely mistaken impression of what they are for, much less how to do them correctly. So I thought I would give away, (free of charge) articles that explain the basics of MSA, as well as a cool MSA spreadsheet to help you learn how to do them, just because that’s the kind of guy I am. Selfless. And humble. Yep.