Using Rational Sub-Grouping to Control Process

Instructor: Scott Tuning

Scott has been a faculty member in higher education for over 10 years. He holds an MBA in Management, an MA in counseling, and an M.Div. in Academic Biblical Studies.

A good process should result in reliable, homogeneous outputs consistent with the company's goals. A rational sub-group is a process control method that evaluates outputs for their degree of variance.

Process Control at Saturn

In the late 1980s and early 1990s, the US auto industry wasn't exactly in the position it is today. Whether justified or not, US automakers during this time period suffered from a perception of inferior quality. In short, many consumers considered foreign-made vehicles, particularly Japanese brands, to be of superior quality and therefore longevity when compared to their American-made counterparts.

In an effort to control process to improve quality, many US automakers, including Saturn, began to integrate many Japanese manufacturing practices into their production lines. Quality and process control concepts like Lean manufacturing were introduced to the processes of many American companies during this time.

On the Saturn assembly line, employees could halt the production process immediately if they observed automated error that posed a quality or safety risk.
Automobile Assembly Line

Process Control

Process control is the mechanism by which a company ensures that its processes are standardized to the point that variance is minimal and there is high reliability in the outputs of the process. In manufacturing, the idea behind process control is making sure that error isn't automated. At Saturn, the idea was to tightly control the assembly line process so that any error or variance that could impact quality or safety could be stopped and corrected immediately - before a single additional product rolled off the line. Measuring the variance between things that should be the same is an important way to determine if the process being used is actually reliably producing its desired output.

But how does a company actually measure whether they are reducing variance?

Rational Sub-Grouping

When comparing the results of process, particularly the production process, it's common to measure the differences between data points. Statistics like production cost per unit, units produced, or error rates are all data points that most companies would watch carefully. Rational sub-grouping is different. Instead of comparing data points over time, the sub-grouping actually examines the results of production that occurred under substantially similar conditions.

Another way to think about rational sub-groups is to think about measuring what should be the same. In the case of Saturn, some variance is expected, but the aspects of the process that vary should not vary within the same assembly line. In other words, it's acceptable that (a fictional) assembly line A (in China) might produce at a different rate than assembly line B (in Sweden). Comparing factory A and factory B production stats is the comparison of data points whose variances are both expected and explainable. But the outputs within assembly line A should not vary widely.

When the same process is used to manufacture or produce, the result should be that the final product is nearly identical to the one made before it, and the one made after it. Applied to our example, good process control means that a vehicle manufactured on assembly line A at 09:05 should be substantially similar to one manufactured on the same line, on the same day, at 15:06, 17:10, and 21:09. In this instance, the four vehicles identified as having been manufactured on the same day would be the rational sub-group.

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