When randomly drawing samples from different population subgroups, you may use stratified sampling. Most people working on political surveys or human research use this sampling method. This article explains what the stratified sampling method entails and gives a step-by-step guide on conducting one.
Definition: Stratified sampling
Stratified sampling came from the word “strata” or “groups.” Thus, stratified sampling usually distributes the accumulated data into multiple subgroups with similar attributes. The researcher picks random samples from the subgroups, analyses the data, and draws different conclusions.
The primary function of this method is to ensure that each sample represents a part of the entire population. This is possible because the process categorizes the whole population into different strata.
Using stratified sampling correctly
It’s crucial to note that each subgroup in a stratified sampling should be mutually exhaustive and exclusive. As such, every member of a population should fit into precisely one subgroup because duplicating data in multiple strata may give unreliable results.
Stratified sampling is effective when choosing an ideal probability sampling method.1 This is because it may give you different mean values on the variables you’re analyzing. The stratified sampling method has potential advantages, such as:
✓ Ensures diversity. A stratified sample includes all subgroups, showing the variety of that population.
✓ Ensures a similar variance level. To have the same level of contention for all subgroups, you should collect the same sample size for all subgroups.
✓ Minimizes the overall variance. Stratified sampling becomes more homogenous with specific subgroups even when the entire population is heterogeneous.
✓ Allows a range of data collection methods. Stratified sampling entails the use of various data collection methods from different strata.
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1. Stratified sampling: Defining the population
The first step of any sampling method is clearly defining the population you’ll study. Stratified sampling is no different, and it takes the following forms:
Choosing characteristics for subgrouping
Once you’ve defined your population, explain what the subgroups would look like. Choose the characteristics that will differentiate your subgroups and divide the population into the relevant groups.
When choosing the characteristics, it’s essential to note that each population member belongs to a single subgroup. Also, the classification of each item to each subset should be clear and conspicuous.
You can also divide your population using multiple characteristics if you group every member into one subgroup. However, to get the overall number of subgroups, you should multiply the number of subgroups for each factor.
2. Stratified sampling: Separating the population
Next, assort all members and group them into a stratum. Ensure that each stratum has no overlap, is mutually exclusive, and represents the entire population when put together.
When you combine these characteristics, you get nine groups in total. You must assign each employee to one subgroup, as shown in the table below:
|Gender identity||• Male
|1. Female employees in London
2. Male employees in London
3. Other employees in London
4. Female employees in West Yorkshire
5. Male employees in West Yorkshire
6. Other employees in West Yorkshire
7. Female employees in Manchester
8. Male employees in Manchester
9. Other employees in Manchester
|Working in one of
the three branch offices
• West Yorkshire
3. Stratified sampling: Deciding the sample size
Thirdly, decide the nature of your sample, as follows:
Proportionate vs. disproportionate sampling
The main difference between the two sampling methods is described in the table below:
|Each stratum's sample size equals the proportion of the subgroup in the entire population.||The sample size is disproportionate to their representation of the entire population.|
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Next, decide your total sample size, which must adequately draw practical statistical conclusions from each subgroup. Use the sample size calculator to estimate numbers when you have errors, such as the desired margin error, standard deviation, confidence level, and estimated size.2
4. Stratified sampling: Random sampling
Lastly, you can use simple random sampling for items within each stratum. Using random sampling in a stratified sampling method allows you to get samples representing that particular subgroup.3
In this method, all target population members have the opportunity to be incorporated in the sample.
When using the stratified sampling method, selecting the appropriate strata for a sample and evaluating and arranging the results is quite tricky.
Every sample should have four to six stratification variables and no more than six strata. Using more variables may increase the chance of some variables cancelling the impact of other variables.
1 Institutional Effectiveness and Assessment. “Probability Sampling.” St. Olaf College. July 30, 2013. https://wp.stolaf.edu/iea/probability-sampling/.
2 Calculator.net. “Sample Size Calculator.” Accessed January 5, 2023. https://www.calculator.net/sample-size-calculator.html.
3 PennState Eberly College of Science. “Lesson 6: Stratified Sampling.” Accessed January 5, 2023. https://online.stat.psu.edu/stat506/book/export/html/655.