Stratified sampling is a technique sometimes used when taking samples for tests. When a population/process contain 2 or more groups, then equal samples are collected from each group, e.g. if sampling is done over three shifts.
Procedure when using Stratified Sampling
Mark or separate samples when they are collected, to be able to identify which samples come from which subgroup. That way collected data can be separated into their respective group when samples are inspected.
When creating the graphs from the collected data, keep it stratified by using different colours or symbols for each group. When doing the analysis, each stratified group is analysed separately.
When to Consider Stratified Sampling
Before starting to collect data, it should always be considered if stratification should be used. Once the data is collected it is usually too late, and you might end up having to start over with your data collection.
Below are some examples where it might be required to use stratified data.
- Different machines
- Several shifts or production lines
- Different materials or designs
- If dual suppliers are used
- Several products or brands
- If data is collected across different departments
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