Stratification is where we divide parts into distinct groups based on their source to help us see patterns in the data. For example, we may stratify by machine, line, gage, plant, location within the part, supplier, operator, shift, and so on.
Using stratification, we can access whether the root cause impacts each group in the same way or in different ways. Stratification works because it creates an analysis by breaking a problem into its component parts allowing you to direct your attention to critical factors.
Stratification is the foundation for other tools such as check sheets, histograms, and Pareto analysis. The most common mistake is the failure to stratify on the critical factor of analysis. Stratification will not tell you what to do about the critical factor, but simply identifying it is often the key to solving your problem.