Improvement Insights Blog

People are often confused about the difference between variable and attribute data. Here’s a simple way to tell which is which:

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“Hi, I’m Jay Arthur, author of “Lean Six Sigma Demystified” and QI Macros [software].

“Over the years, I’ve found one of the things people get confused about is what’s ‘variable’ and what’s ‘attribute.’ Those words, for some reason, strike terror in the hearts of newbies, so I’m going to give you my short way of figuring this out: It has to do with decimals and integers.

“If you have a number that has a decimal point in it, it must be measured. It must be variable. It has to be. No other option, because we’ve split it down into smaller parts, that’s the definition of variable. Now, that means that if you see something with decimal points, if it’s a single column, it has to be an XmR or I-mR chart. It has to be an Individual Moving Range chart. End of story. I you have multiple samples per subgroup, well then you might get into an X bar R or an X bar S chart, but most people are not gonna get there unless you’re in manufacturing when you do sampling.

“So, decimals mean variable. That leaves integers, right? So integers are measured, indivisible. They have to be attribute; they have to be attribute. Especially if you have a numerator and a denominator; so number of defects over total number of parts. Number of medication errors over total medications dispensed, right? Any bunch of integers, the smaller one on top and the bigger one on the bottom, is, by definition, attribute. The only place that gets a little confusing is when there’s just one set of integers. Well, it could be things like ‘injuries,’ so that’d be a c chart, but it could also be ‘number of days to pay an invoice,’ and days are measured. That doesn’t have a decimal point, but that could be variable.

“Now, here’s the fun thing. Little known fact: you take the numerator, you divide it by the denominator, you get a ratio, and that ratio has a decimal point, and guess what? You can use an XmR chart with that. So you can convert just about everything there is to know into data you can use with an XmR chart.

“When you’re first learning, master one chart. Dr. Donald Wheeler calls the XmR or I-mR chart ‘the Swiss Army knife of charts.’ Master that one chart and really get used to using it, and then you can begin to add in the others.

“So this is pretty simple, right? Decimals, variable. Integers has to be attribute, and you can convert this into something you can use with an XmR chart. I think we’ve made this entirely too hard and people stumble over the words ‘variable’ and ‘attribute’ when ‘decimals’ and ‘integers’ might be a simpler way to explain it.

“So that’s my improvement insight. Let’s go out and improve something this week, including the way we teach things about different types of data.”