Author's note: To my non-U.S. readers, I apologize for using the sport of baseball to make my points today—and during the World Cup, no less! It’s a perfect context, however, and I hope you will be able to understand the main points.

In my last column, I talked about the different types of control charts and encouraged the use of the individuals chart almost exclusively. I also mentioned that p-charts (percentages) and u-charts (rates) can be very useful for stratification. I’m going to pursue that more in the next couple of columns, but today it’s “back to basics” to set up their foundation by talking about count data.

Count data seem intuitively simple

Let’s begin with a common issue: tracking complaints. How do you count them? Do you count the number of complaints received each month, or do you count the number of customers who complained? The people tallying such data will certainly need careful instruction before you can begin to collect useful count data—lurking “human variation” can seriously compromise the quality of the data and its use.

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