based on gender

After this segregation, samples are selected from each of these strata to mirror the actual population mix. The term stratified emerged from the word “strata,” which refers to groups. Thus, stratified random sampling emphasizes distributing the assorted data into multiple groups. A sample or data set is selected from each of these groups for analysis.

simple random sample

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Stratified sampling can not be applied if the population cannot be completely assigned into strata, which would result in sample sizes proportional to sample available instead of overall subgroup population. In real life, stratified random sampling can be applied to results of election polling, investigations into income disparities among social groups, or measurements of education opportunities across nations. Image by authorUsing stratified sampling, the proportion of the target variable is pretty much the same across the original data, training set and test set.

What Is Stratified Random Sampling?

A random sample from each stratum is taken in a number proportional to the stratum’s size compared with the population. These subsets of the strata are then pooled to form a random sample. Stratified sampling is not useful when the population cannot be exhaustively partitioned into disjoint subgroups. Data representing each subgroup are taken to be of equal importance if suspected variation among them warrants stratified sampling. For an efficient way to partition sampling resources among groups that vary in their means, variance and costs, see “optimum allocation”. The problem of stratified sampling in the case of unknown class priors can have a deleterious effect on the performance of any analysis on the dataset, e.g. classification.

  • And we are asked to take a sample of 40 staff, stratified according to the above categories.
  • Defenestration The fascinating story behind many people’s favori…
  • Optimal stratified sampling is always the same or more accurate than proportional stratified sampling.
  • When the samples are taken in the same percentage or ratio from each subgroup, it is known as proportionate stratified random sampling.
  • A stratified random sampling approach divides the population into relevant strata to increase a certain population group’s representativeness.

Block randomization is commonly used in the experiment with a relatively big sampling size to avoid the imbalance allocation of samples with important characteristics. In certain fields with strict requests of randomization such as clinical trials, the allocation would be predictable when there is no blinding process for conductors and the block size is limited. At minimum, one element must be chosen from each stratum so that the final sample includes representatives from every stratum. If two or more elements from each stratum are selected, error margins of the collected data can be calculated. Cross-validation implemented using stratified sampling ensures that the proportion of the feature of interest is the same across the original data, training set and the test set. This ensures that no value is over/under-represented in the training and test sets, which gives a more accurate estimate of performance/error.

A Stratified Analysis

It is a smart way to ensure that all the sub-groups in your research population are well-represented in the sample. Stratified sampling helps you to save cost and time because you’d be working with a small and precise sample. For many applications, measurements become more manageable and/or cheaper when the population is grouped into strata. Data stratification breaks a large amount of data down into smaller groups of information. Once the data has been categorized, the smaller bits of information can help tell a story of why certain issues may occur.

Thus, the analysis turns out to be more accurate when the variables are selected from all subgroups of interest. Stratified random sampling captures the key attributes of a population group. As a result, it produces characteristics in the sample that are proportional to the entire population. Therefore stratified random sampling provides a higher degree of precision than simple random sampling.

  • The strata will depend on the subgroups in which you are interested that appear in your population.
  • The knowledge of these data in the exposed form, that is, stratified, will allow assessing the convenience of adopting certain actions.
  • It is applied when raising data collections and in the analysis and representation of the data by means of Pareto diagrams, histograms and correlation diagrams.
  • Unfortunately, this method of research cannot be used in every study.

Using the ‘KFold’ class of Scikit-Learn, we’ll implement 3-fold cross-validation without stratified sampling. We’ll implement hold-out cross-validation with stratified sampling such that the training and the test sets have same proportion of the target variable. This can be achieved by setting the ‘stratify’ argument of ‘train_test_split’ to the characteristic of interest . It need not necessarily be the target variable, it can even be an input variable which you want to have the same proportion in the training and test sets.

Stratified randomization in clinical trials

Let’s discuss other differences between stratified sampling and cluster sampling. The number of subgroups can be calculated by multiplying the number of strata for each factor. Factors are measured before or at the time of randomization and experimental subjects are divided into several subgroups or strata according to the results of measurements.

Whether you opt for proportionate or disproportionate stratified sampling, the most important thing is creating sub-groups that are internally homogenous, and externally heterogeneous. This way, you can account for minority groups and have a truly representative sample. Disproportionate stratified sampling means the researcher randomly chooses members of the sample from each group. So, you could have 60,000 participants from the first group and 20,000 and 17,000 from others, respectively.

sampling involves

Hold-out cross validation is implemented using the ‘train_test_split’ method of Scikit-Learn. Deciding how far down you want to stratify before collecting data will inform you of the amount of data you’ll need to collect to do a statistically valid analysis. Don’t wait until after you have your data to decide how much stratification that you want.

Statistics is the collection, description, analysis, and inference of conclusions from quantitative data. Sampling involves statistical inference made using a subset of a population. Full BioKatharine Beer is a writer, editor, and archivist based in New York. She has a broad range of experience in research and writing, having covered subjects as diverse as the history of New York City’s community gardens and Beyonce’s 2018 Coachella performance.

What is Structured Data?

Typically, the researcher derives a sampling fraction and uses this fraction to determine how the variables are selected for the sample. This sampling fraction is always the same across all strata, regardless of their sizes. With disproportionate stratified sampling, every unit in a stratum stands the same chance of getting selected for the systematic investigation. A real-world example of using stratified sampling would be for a political survey. A stratified survey could thus claim to be more representative of the population than a survey of simple random sampling or systematic sampling. A stratified sample includes subjects from every subgroup, ensuring that it reflects the diversity of your population.

Then, using the disproportionate method, the researcher selects 600 people from category A and C and 800 people from category B. Then based on these 1000 opinions you can’t decide the opinion of that entire state on your product. In a stratified sample, individuals within each stratum are selected randomly, while in a quota sample, researchers choose the sample instead of randomly selecting it. A sample is the participants you select from a target population to make generalizations about.

Stratified sampling strategies

You might choose this stratified data definition if you wish to study a particularly underrepresented subgroup whose sample size would otherwise be too low to allow you to draw any statistical conclusions. In this case, stratified sampling allows for more precise measures of the variables you wish to study, with lower variance within each subgroup and therefore for the population as a whole. Portfolio managers can use stratified random sampling to create portfolios by replicating an index such as a bond index. The team then needs to confirm that the stratum of the population is in proportion to the stratum in the sample; however, they find the proportions are not equal.

Mutually 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. The ZZ-400 manufacturing team drew a scatter diagram to test whether product purity and iron contamination were related, but the plot did not demonstrate a relationship. Then a team member realized that the data came from three different reactors. The team member redrew the diagram, using a different symbol for each reactor’s data .


When are picked up in no prescribed ratio or rate, it is referred to as disproportionate stratified random sampling. Representative means the extent to which a sample mirrors a researcher’s target population and reflects its characteristics (e.g., gender, ethnicity, socioeconomic level). In an attempt to select a representative sample and avoid sampling bias (the over-representation of one category of participant in the sample), psychologists utilize a variety of sampling methods. It only works under the condition where a population can be stratified using relevant attributes and that the subgroups are clearly defined and do not overlap.

In general, simple random sampling is often the easiest and cheapest, but stratified sampling can produce a more accurate sample relative to the population under study. Proportional stratified random sampling involves taking random samples from stratified groups, in proportion to the population. In disproportionate sampling, the strata are not proportional to the occurrence of the population. Stratified random sampling is a method of sampling that involves the division of a population into smaller subgroups known as strata. In stratified random sampling, or stratification, the strata are formed based on members’ shared attributes or characteristics, such as income or educational attainment.


Conducting research on the level of education amongst women in a community, one can identify different population groups based on ethnicity, gender, religion, and income level. The whole idea is to preserve the homogeneity within each group, so that no subset is excluded from the eventual sample. Within each stratum, several randomization strategies can be applied, which involves simple randomization, blocked randomization, and minimization. Stratified randomization may also refer to the random assignment of treatments to subjects, in addition to referring to random sampling of subjects from a population, as described above. Each stratum should be mutually exclusive and add up to cover all members of the population, whilst each member of the population should fall into unique stratum, along with other members with minimum differences.

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