Seven Quality Tools – Stratification

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Stratification is a technique used to divide a population into homogeneous subsets.

Studying defect types for multiple products might not give you a good idea about the cause. For example, you may want to study the types of defects produced by a production line making multiple types of products. You could stratify your data so that every product has its own subgroup and perform statistical analysis on them separately.

The basic idea behind this technique is that if you can identify groups within the data, you have more meaningful information than just looking at the whole group.

What Is Stratification?

Stratification is one of the Seven Basic Quality Tools.

It works by dividing a population into homogeneous subgroups. In other words, it's a way of breaking down a large population into smaller groups of similar nature.

In general, you would use stratification to know which part of the population is doing well or poorly. You won't get any helpful information if you're only studying the entire population. For example, you may be interested in learning how many defects were found during the production of three sizes of water bottles. By stratifying the data, you'll get an idea about what type of bottle is getting a particular kind of defect more.

What is Stratification Used for?

Stratification is used for :

- determining whether there are differences between groups

- determining whether the problem being investigated is only limited to a specific subgroup only

- determining whether the problems are similar across different groups


What Are the Good Features of Stratification?

The following features make stratification an excellent investigation tool.

1. It allows you to see the difference between groups.

2. It helps determine whether the problem is confined to a specific subgroup.

3. It gives you insight into why the problem exists.

4. It lets you compare groups with respect to some aspects.

5. It makes it easier to understand results.


Tools for Stratification

Unlike other quality tools (such as Histogram, Cause and Effects Diagram, Pareto Chart, Control Chart), this tool is not a graphic tool.

Instead, this is an analytic tool where you manipulate data by dividing it into subgroups and studying the characteristics of that subgroup. You can use other tools such as Histogram along with Stratification. For example, instead of looking at the Histogram of the complete data set, you might want to draw separate Histograms of groups created as part of Stratification.

To use stratification as one of the seven basic quality tools to solve a quality problem, you need to have a good understanding of the data and data analysis tools.

Various tools can be used for stratification.

1. Manual: Using a pen and paper: This method can work only if the data is small.

2. Microsoft Excel: This is the most common tool used for stratification.

3. Minitab: Minitab is advanced statistical software Six Sigma professionals use. You can use Minitab to divide data into subgroups.

4. Python: Python is a programming language that can be used for many different purposes. It is one of the most popular languages used today for data analysis.

5. R Programming: R is a programming language used for complex statistical analysis. Like Python, you can use R Programming for data analysis.

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