Stratification: A Basic Quality Tool
Stratification implies layers or differences. A quick test for soil composition is to place a sample of soil with water in a clear jar and give it a shake. The sand, silt and loam will settle at different rates and create a layered appearance within the jar over time. The height of each layer provides information about the proportion of each type of soil within the sample.
Stratification as one of the seven basic quality tools (some lists use a run chart or flowchart instead) the idea of layers or differences still applies. The idea is to identify potentially meaning differences within a sample set.
Reasons to Use Stratification
When we are faced with a set of data we often want to understand the information contained within the dataset. A common technique is to plot the data using histogram or scatter plot. Yet at times, those are not informative. about potential underlying common causes of differences within the data.
The primary reason to use stratification is to learn a little more about the data. For example, what causes the higher/lower values? What separates good items from out of spec items? What are the significant sources of variation within our process? and so on.
We may be exploring difference, and want to initially detect if a difference exists, between shifts, different factory lines or locations, different materials, amount suppliers for a similar part, time of day or week, seasonal differences, etc.
Identifying differences that relate to some factor may allow eliminating or reducing the variability. Or, it may identify a potential root cause source for unwanted variability. Or, the identified differences may lead to calibration, measurement system, or alignment problems within the system.
Stratification can also identify new product segmentation or solutions for customer problems.
In short, stratification is a tool that allows us to better explore and understand data.
The Basic Stratification Process
Back to the soil testing process, if presented with a dozen soil samples and their testing…