A control chart is a tool used to interpret data from a system or process over time. There are a variety of control chart types, and selecting the right type is essential for accuracy. If the wrong chart type is selected, misleading analysis may occur. Determining the chart type is a straightforward process that depends on the format of data being plotted and the sample or subgroup size in which it is collected.

Step 1, look at the format in which the data is collected. It is either in variables (measurement data) or attributes (count data) format.

Variables data is defined as a measurement. Examples are height, weight, time, or length. If you use a measurement device, such as a scale, gage, or clock to collect the data, it is most likely variables data. Another characteristic of variables data is that it may contain decimal places.

Attributes data is defined as a count, such as the number of defective products, the number of errors, the number of employees, or the number of complaints. The count data can potentially be larger than the subgroup. For example, one customer or shipment may have several complaints, or one purchase order might have many errors. Attributes data never contains decimal places; it is always a whole number.

Step 2, look at the sample or subgroup size, which is defined as the amount of data collected at one time, for example:

When assessing the temperature in a vat of liquid, the reading is measured once hourly; therefore, the sample size is one per hour.



When measuring the height of a part, five parts are selected and measured every 15 minutes, so the sample size is five.



When checking ten invoices per day for errors, the sample size (number inspected) is 10.

Step 3, once the type of data and the sample size are known, use the following flowchart to choose the most appropriate chart.

© Cleary, M., and Graham, J. Practical Tools for Continuous Improvement – Volume One (Dayton, OH: PQ Systems, Inc. 2000)