Stratified Sampling

Stratified Sampling Definition

Stratified sampling is a random sampling method of dividing the population into various subgroups or strata and drawing a random sample from each. Each subgroup or stratum consists of items that have common characteristics. This sampling method is widely used in human research or political surveys.

Stratified Sampling

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It is different from simple random sampling that directly takes any part of the entire population as a sample. Moreover, it is a reliable technique since the items exhibiting different features are equally selected to form a sample that mirrors the whole population. Thus, such a sample is a better representative of the overall population.

Key Takeaways
  • Stratified sampling is a process whereby the heterogeneous population is segregated into various homogenous subgroups or strata, and a sample is extracted from each.
  •  A “stratum” is nothing but a group; it is plurally written as strata. Thus, stratification is the process of grouping items or data.
  • Stratified sampling can be proportionate or disproportionate. When the samples are taken in the same percentage or ratio from each subgroup, it is known as proportionate stratified random sampling.
  • When samples are picked up in no prescribed ratio or rate, it is referred to as disproportionate stratified random sampling.
  • The stratified sample is more reflective of the whole population as each subgroup is adequately represented in the sample.

How Stratified Sampling Method Works?

The term stratified emerged from the word “strata,” which refers to groups. Thus, stratified random sampling emphasizes distributing the assorted data into multiple groups. Each group has variables of similar attributes. A sample or data set is selected from each of these groups for analysis.

Please note that each stratum must be mutually exclusiveMutually ExclusiveMutually exclusive refers to those statistical events which cannot take place at the same time. Thus, these events are entirely independent of one another, i.e., one event's outcome has no impact on the other event's result.read more and exhaustive. In other words, an element or item included in one stratum cannot be added to any other stratum. Duplication of data in multiple strata may lead to unreliable results.

The primary purpose of this technique is to ensure that the total sample is a blend of all the different kinds of items in the population. This mix guarantees that the whole population is closely replicated in the sample.

Let’s assume a research team is surveying an FMCG company about the taste and preferences of people in food choices. The team decided to take three significant categories: men, women, and children. The total number of persons required for the data set is close to one million in numbers.

How could stratified random sampling help researchers gather the data needed using less time and resources? It isn’t easy to talk to one million people and take their opinion. However, it’s much more convenient and time-saving to create three groups and select a few amongst them, say 10% people from each group.

The selected individuals will represent their group in the sample.  Their opinion will be similar to most of the individuals in their group. Thus, sampling through data segregation will ensure each category or group is sufficiently represented in the sample. Therefore, the survey results will speak for the whole population.

This sampling method has been a standard probability sampling technique used by portfolio managersPortfolio ManagersA portfolio manager is a financial market expert who strategically designs investment portfolios.read more to design portfolios for their clients. It provides the desired returns by replicating different indexes like the stocks index or the bonds index.

Also, it is a prominent practice in auditing and vouching. Even an auditorAuditorAn auditor is a professional appointed by an enterprise for an independent analysis of their accounting records and financial statements. An auditor issues a report about the accuracy and reliability of financial statements based on the country's local operating laws.read more, generally Certified Public Accountant (CPA), uses this formula at large for vouching and verification purposes in auditing the company’s accounts. This formula fits well for their criteria as auditors can create various groups or subgroups based on the amounts involved. This practice helps reduce the sample size without compromising on the reliability of the sample collected.

Types of Stratified Sampling

There are two fundamental ways of executing this sampling technique. These are as follows:

#1 – Proportionate:

Here, the same percentage of items is selected from each stratum. The sample size of each stratum is proportional to its population. The total of the samples from all groups forms the total sample size of the whole population.

For example, suppose the population of a town has to be divided into three categories based on their age.

Category Age Limit Population
ABelow 18 years4100
B18 – 44 years3500
C44 years above2400
Total 10000

If the sample size is 2000, we can determine the number of samples taken from each group using proportionate sampling.

Proportion of sample size to population = 2000/10000*1000 = 20%

Category Population Sample
A410020% of 4100 = 820
B350020% of 3500 = 700
C240020% of 2400 = 480
Total1000020% of 10000 = 2000

Sample size = 820 + 700 + 480 = 2000

In the above illustration, we observe that 20% of sample items are selected from each category. Also, the cumulative number of samples taken from all the subgroups combine to form 20% of the total sample size.

#2 – Disproportionate:

Here, the size of each stratum is not proportional to its population size. The researcher doesn’t take the samples in the same ratio from each group under this random sampling techniqueRandom Sampling TechniqueSimple random sampling is a process in which each article or object in a population has an equal chance of being selected, and using this model reduces the possibility of bias towards specific objects.read more. Thus, the sample selection may not be equitable in this case. For instance, the researcher can select the same number of items from each stratum irrespective of the group size.

Going by the above example, suppose the sample size remains 2000 people. Then, using the disproportionate method, the researcher selects 600 people from category A and C and 800 people from category B.

Category Population Sample
A4100600
B3500800
C2400600
Total100002000

So here, the researcher has picked samples regardless of the population size of each stratum. Thus, even though category A has the maximum population size and category C has the lowest population size, their sample size is the same.

Stratified Sampling Formula

There is no particular formula for this sampling since the decisions like division of sub-groups or strata and the total sample size to reflect the entire population are at the discretion of the researcher. 

But the following formula can be used to find out the sample size for each subgroup under the proportionate sampling:

Stratified-Sampling-Formula

Stratified Sampling Example

A business research team has to survey 120,000 employees working in different U.S. locations of a company. The number of employees employed in various branches of the company is as follows:

Branch Office Number of Employees
Washington20,000
New York26,000
New Jersey19,000
California38,000
Kansas17,000
Total120,000

If the total sample sizeSample SizeThe sample size formula depicts the relevant population range on which an experiment or survey is conducted. It is measured using the population size, the critical value of normal distribution at the required confidence level, sample proportion and margin of error.read more is 12,000, the team can determine the samples from each stratum or sub-group using the following formula.

Stratified Sampling = Total Sample Size / Entire Population * Population of Subgroups

Calculation of the sample size for the Washington office:

Number of Samples = (12,000/120,000) *20,000

Sample Size of Washington Office = 2,000

Similarly, we can find the sample size for all branch offices using the above formula.

Branch Office Sample Size
Washington2,000
New York2,600
New Jersey1,900
California3,800
Kansas1,700
Total12,000

Real-life Example

research paper published in medRxiv discusses the suitability of using the stratified random sampling technique for estimating COVID-19 prevalence in the U.S. state of Maryland. In this survey, the population of Maryland was stratified or divided based on counties. Then, individuals were selected from each county representing their stratum.

As per the study, the stratified sampling technique for testing COVID-19 prevalence is acceptable. But the sample arrived through stratification must be adjusted for misclassification error to avoid under-or overestimation of COVID cases.

Advantages

This sampling technique is a highly effective technique for the following reasons:

  • Replicates Heterogeneous Population: It is efficient to select a sample of varying characteristics by creating subgroups. Thus, the samples from each subgroup or stratum effectively represent the entire population.
  • Fair Analysis: It includes the samples with distinct data giving a reasonable weightage to each category for unbiased interpretation.
  • Accurate and Reliable Results: When the samples are evenly taken from all the categories or groups with different attributes, it tends to provide efficient and meaningful outcomes.
  • Saves Time and Money: Studying the whole population is tedious and leads to the wastage of resources. At the same time, this technique helps select a very proximate sample of significantly smaller size, which saves the researcher time and money.
  • Facilitates Comparative Study: It clearly distinguishes the entire population into different strata by its features. Therefore, the data of each of these groups can also be compared and analyzed separately.

Disadvantages

Undoubtedly, this random sampling technique simplifies the process of research or analysis.  However, it is subject to errors and inaccuracies. Let’s discuss some of the limitations that confine its applicability:

  • Limited Scope: This method becomes invalid in the absence of consolidated information regarding the various attributes and mix of the population. Thus, it cannot be applied to every kind of study.
  • Difficulty in Deciding Strata: Another significant problem is the formation of categories or groups. Identifying what to include or exclude and what characteristics to be considered is another challenge.
  • Inapplicable to Small Population Size: When the population size is limited, say 100 people or so, there is no need for sampling. Instead, the whole population can be considered for analysis.
  • Prone to Biasness: Further, this method is highly influenced by the researcher’s selection of groups, which at times may not be fair enough. Also, the mindset and abilities differ from person to person, which may affect the sampling.

Stratified Sampling vs. Cluster Sampling

Both stratified and cluster samplingCluster SamplingCluster sampling is a cost-effective method in comparison to other statistical methods in which researchers distribute the population into individual groups known as clusters and select random samples from the population to analyze and interpret results rather than looking at the entire set of available data.read more are random sampling techniques. In stratified random sampling, different subgroups are formed, and each of these has items with the same attributes. After this segregation, samples are selected from each of these strata to mirror the actual population mix.

On the contrary, cluster sampling is also the process of dividing the entire population into subgroups. However, heterogeneous groups are formed where each cluster is a mix of items with different attributes. In this method, random cluster(s) are chosen and their elements form the final sample. Here, the cluster is taken as a sample as it replicates the total heterogeneous population.

In the former, the groups are called strata, while in the latter, these are termed clusters. Also, the sample in stratified sampling is the elements in the strata, whereas, in cluster sampling, a cluster or group is considered a sample. In the former, the researcher forms heterogeneous strata, each with homogenous items. However, in the latter, the researcher makes homogenous clusters with heterogeneous items. 

Frequently Asked Questions (FAQs)

What is stratified sampling?

Stratified sampling refers to a random sampling technique that clubs items of the whole population into different groups based on their similar characteristics. Then, samples from each stratum are taken, whether proportionately or disproportionately, to conduct the research or analysis.

Why is stratified sampling used?

The stratified random sampling method provides sample data that is almost identical to the entire population data. Thus, the analysis turns out to be more accurate when the variables are selected from all subgroups of interest.

Is stratified sampling non-probability sampling?

No, instead, it is a probability sampling technique. Also termed stratified random sampling, this form of sampling is used in research to determine the entire population’s possible behavior efficiently.

What are the four types of probability sampling?

Given below are the four different kinds of probability sampling:
#1 – Simple random sampling
#2 – Stratified sampling
#3 – Systematic sampling
#4 – Cluster sampling

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