ARTICLE 38: Research Methods for Ph. D. and Master’s Degree Studies: Sampling Part 6 of 6

Written by Dr. Hannes Nel


Some academics and other scientists are of the opinion that the COVID-19 tests can be inaccurate by as much as 30 percent.

And there are those who believe that the mortality rates due to COVID-19 in some countries are much higher than the reported figures.

Perhaps it is true that findings and statistics are sometimes deliberately manipulated.

Then again, imagine how near impossible it must be to conduct research on an issue that affects the entire world.

Researchers often have no other choice than to make use of sampling.

And sampling facilitates efficiency and viability, not validity and accuracy.

I will discuss typical case sampling, unique-case sampling and volunteer sampling in this article.

Typical-case sampling

You can use typical-case sampling when you need to collect data on general items.

A typical-case sample is composed by identifying and including people who can be regarded as typical for a community or phenomenon.

Such a sample is used to avoid information, or worse, research findings and recommendations being rejected because they have been obtained from suspect, perhaps even deviant, cases.

It can also be used to avoid using data or opinions that are politically laden or otherwise manipulated.

A typical case sample should allow you, as the researcher, to develop a profile of what would generally be agreed as being average or normal.

Typical-case samples are often useful when large communities and complex problems are investigated.

Such samples enable those who are not familiar with a community to gain an understanding of the nature of the community in a relatively short time.

Members of the community can often help you to select items for a sample by suggesting criteria for such samples.

An example of a typical-case sample is when you choose your sample from a middle-class suburb rather than from a poor or rich suburb if you wish to do research on the spending habits of a city.

The spending habits of the poor and the rich would probably not be typical for the community.

Of course, your sample will be more representative of the community if you were to include poor, middle class and rich people in your sample.

But then you would be using random sampling or systematic sampling.

Unique-case sampling

In unique-case sampling, you would identify and include cases that are rare, unique or unusual in terms of one or more characteristics.

Such cases might agree with typical cases in other respects.

An example of unique-case sampling can be if you were to do research on the genetic makeup of a nation or tribe that has not been affected by die COVID-19 virus, or has been affected markedly less that the average for other countries.

Volunteer sampling

As in the case of snowball sampling, you will need to take intentional steps to gain access to individuals who are difficult to establish contact with.

Also, as in the case of snowball sampling, you might need to rely on volunteers for your sample group.

Then again, most research rely on the co-operation of volunteers.

You would typically start by approaching family members, friends, friends of friends, etc.

You can also ask for volunteers by posting an advertisement in a newspaper, ask for help on social media, write letters to possible volunteers, visit managers of businesses and ask them for assistance, etc.

Making use of volunteers whom you do not know as your sample can be risky.

You often do not have any control over the validity and accuracy, authenticity or sincerity of their responses to questions or any other contributions to your research.

It will, furthermore, be rather difficult to claim that your findings apply generally if you do not know the motives or reasons why volunteers became involved in your research project.


Typical-case sampling can be used with good effect to avoid using subjective data. This is done by collecting data from a general population, items or phenomena.

Unique-case sampling is used to investigate rare, unique or unusual cases or phenomena.

In volunteer sampling people are asked to volunteer to be included in the sample.

Volunteers need to be screened to ensure that they are authentic and trustworthy.

This is my last video on sampling, Methods. Therefore, I guess it would be fitting if I were to summarise the links between the different types of sampling. Here it is:

  1. Cluster sampling is a form of stage sampling.
  2. Convenience sampling is a form of opportunistic sampling.
  3. Convenience sampling, dimensional sampling, quota sampling, purposive sampling and snowball sampling can be conducted as non-probability sampling.
  4. Extreme-case sampling is used in conjunction with most other types of sampling.
  5. Matched sampling and stratified sampling use systematic sampling.
  6. Multi-phase sampling is a form of stage sampling.
  7. Purposive sampling is a variant of boosted sampling.
  8. Reputational sampling and unique-case sampling are variants of extreme-case sampling.
  9. Snowball sampling is a form of volunteer sampling.
  10. Systematic sampling is a modified form of random sampling.
  11. Theoretical sampling is a form of criterion sampling and dimensional sampling.
  12. Time sampling includes instantaneous sampling, interval sampling and duration sampling.
  13. Typical-case sampling is related to case-study sampling.
  14. Typical-case sampling is the opposite of critical-case sampling.
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ARTICLE 37: Research Methods for Ph. D. and Master’s degree Studies: Sampling Part 5 of 6

Written by Dr. Hannes Nel


On face value sampling seems like a simple job.

And so it mostly is.

However, if you choose the wrong sample or you choose the sample incorrectly, you can destroy the validity and accuracy of your research project.

And you might have wasted a good number of years on fruitless research.

Think carefully about the examples that I offer in this post, especially the ones on systematic sampling.

I will discuss:

  1. Stratified sampling.
  2. Systematic sampling.
  3. Theoretical sampling.
  4. Time sampling.

In this post.

Stratified sampling

Stratified sampling is used to ensure that the sample is representative of a population with multiple characteristics that are evenly distributed amongst the population.

The research population is divided into homogeneous groups, with each group containing subjects with similar characteristics.

Such homogeneous groups are often called ‘sub-units’ or ‘strata’.

Samples are selected by making use of random selection procedures such as systematic sampling or simple random sampling.

For example, students at a specific university, which are the population on which the research will be conducted, can be grouped into a male and a female group.

Or they can be grouped per faculty.

Or per leisure time interests, etc.

If, for example, you wish to do research on the impact of leisure time interest on academic performance, you will need to investigate a representative sample from each leisure time interest group.

Representivity is achieved by identifying the parameters of the wider population and taking them into consideration when composing samples.

So, in this example leisure time utility will be the independent variable.

Factors such as gender, age, occupation, academic year, etc. can be the dependent variables.

For the sake of coming to valid conclusions and findings, the process of organizing a stratified sample requires ensuring that the characteristics typical of the wider community are present in the sample.

If you draw more than one sample, you must also ensure that the samples are homogeneous in terms of at least one characteristic, which would be your independent variable.

In qualitative research, you will probably not calculate the size of the sample in terms of a statistical level of accuracy.

Therefore, representivity will depend on your own judgement.

You do need to take certain factors into consideration, though.

For example, the size of the population, time and cost constraints, access to data, the need for simplicity and the purpose of the reach.

Systematic sampling

Although some simple calculations need to be done with systematic sampling, it is not enough to be called quantitative research.

Systematic sampling is a modified form of random sampling.

For example, in a population of 10,000 individuals, you might feel that you should have a sample of, say, 200 people.

This will mean that you should select every first person from each sub-unit of 50 for your sample.

Fifty is, thus, your sample interval, i.e. the distance between each element selected for the sample.

You can choose numbers 1, 51, 101, 151, etc. for your sample.

If, for example, the first 57 people on the list that you use are the only females, you will have only two females in your sample.

This will probably distort your findings.

Another example of a skewed list would be if you work on a number of name lists for different faculties and the faculty lists are ordered normatively with the best performing student as the first name on the list and the poorest performing student as the last name on the list.

To use an extreme example, if there are fifty names on each list, your sample will include only the poorest performing students.

Periodicity can be a problem with systematic sampling if the list is not a fully random one.

For example, if you start by selecting the first name on the list, then names numbered 1 to 49, 50 to 99, 100 to 149, etc. do not have any chance of being selected for the sample.

You can eliminate this problem by ‘randomizing’ the list first by ‘shuffling’ the names like you would a deck of playing cards, and then selecting the sample.

You can also deviate somewhat from a systematic selection. A sampling procedure is not cast in concrete and you can adapt it to your research needs.

Just as long as it improves the randomness of your sample.

You can, for example, use systematic sampling coupled with the condition that both genders are fairly represented in the sample.

This can be achieved by splitting the name lists for males and females and then selecting a number of males and females from each list randomly.

Theoretical sampling

Theoretical sampling is the process of investigating incidents, events, occurrences or people over a period based on their potential to represent or demonstrate important theoretical constructs.

Researchers who follow an approach where deductions can be made from data with which theory can be developed or extended would be interested in finding individuals with the right makeup, or cases that embody theoretical constructs.

Theoretical sampling, therefore, is a special type of criterion sampling.

As the name implies, theoretical sampling is best used when the research focuses on theory and concept development.

Your goal would be to develop theory and concepts that are connected to, are grounded in, or emergent from real life events and circumstances.

To generate new theory will often require substantial research and is, therefore, a time-consuming process.

You will need to analyze and probably reconstruct and deconstruct existing theory until you manage to develop new theoretical ideas.

As opposed to purposeful sampling, you cannot know in advance precisely what to sample for and where it will lead you when you use theoretical sampling.

That is why theoretical sampling fits in well with grounded theory and why you will probably work towards solving a problem statement or question rather than hypothesis testing.

Time sampling

Time sampling is where observations of occurrences or events need to be taken and recorded at certain times.

You can use it if it is important to know the chronology of events.

Time sampling can be instantaneous, at specific intervals or measured in terms of how long it takes, i.e. duration.

Instantaneous sampling is achieved by observing an event or occurrence at standard intervals, for example every minute, hour, etc. on the dot.

You will need to take observations at a specific time and intervals that you planned on, or that are prescribed or necessary for whatever reason and you will take notes of your observations.

In the case of interval recording, you will observe and record events or occurrences during each interval.

This means that you will record observations during each interval for the entire time of the interval and not just at the moment each interval starts.

For example, you might wish to conduct research on the emotional state of a patient over a period and at set intervals, say every fifteen minutes for a day.

You can then count the number of times that the patient is happy, sad, angry, etc. and how the patient expressed the emotions.

The patient might have smiled, cried, laughed, etc.

You might also need to take note of what possibly might have triggered the emotion.

Duration sampling is where you will measure how long it takes for an individual or group to complete a certain task.

With duration sampling, you might, for example, need to record how long it takes the patient to overcome an anger attack or any other emotional state.

Or you might need to record how often the individual switches between different emotions, and many more.

The format of your notes will depend on the type of data that you wish to collect.

Time sampling will often require a quantitative research approach or at least some calculations.

This, however, does not mean that it cannot be used in qualitative research.

It might also be necessary to use a mixed approach, i.e. quantitative and qualitative research.


Stratified sampling is where you will select a representative sample from a diverse community. You will use random sampling to select the sample or samples.

In systematic sampling you will select individuals or items from a population at structured intervals.

In theoretical sampling you will select the sample based on certain criteria that will enable you to develop new theories and concepts.

Time sampling is where you will take samples at certain times. It can be instantaneous, at specific intervals or measured in terms of duration.

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ARTICLE 36: Research Methods for Ph. D. and Master’s Degree Studies: Sampling Part 4 of 6

Written by Dr. Hannes Nel


Sampling types mostly differ only in the way in which the sample is composed.

Ultimately, samples are all small extracts from a larger population.

They are studied because the entire population is too large for viable research.

Obviously, the sample should display behaviour, phenomena, events, etc. that would be typical for the entire population.

That means that the research findings based on the sample should be generalizable.

This, however, is not always the case.

Researchers sometimes limit their research to the sample without even trying to achieve generalizability.

In effect, what is supposed to be a sample becomes the population.

I will discuss the following sampling methods in this article:

  1. Probability sampling.
  2. Purposive sampling.
  3. Quota sampling.
  4. Random sampling.
  5. Reputational-case sampling.
  6. Snowball sampling.
  7. Stage sampling.

Probability sampling

A probability sample is drawn randomly from the wider population.

Therefore, it will be useful if you need to generalize.

Or if you seek representativeness of the wider population.

The name already reveals that it should be used with quantitative research.

Obviously, statistical calculations will form part of the research process.

Probability or representativeness is calculated with a wonted measure of accuracy probability.

For example, 97% (giving an error factor of 3%).

The risk of bias is less than if you were to rely on your gut feeling or subjective judgement.

However, all samples, regardless of whether they are based on statistical calculation or your judgement, are hampered by a measure of sample error.

Purposive sampling

Purposive sampling is also sometimes called purposeful sampling or judgmental sampling.

In purposive sampling the researcher collects data according to the aims of the research.

To develop a purposive sample that will sufficiently represent the population:

  • You will need to know the profile of your target group.
  • And you will select items for the sample according to a list of important characteristics.

You can select purposive samples after you have studied the bigger population from which to select the sample.

This might require fieldwork or studying written profiles, such as curriculum vitae.

It can also require fieldwork and literature study.

You will need to find certain types of individuals displaying attributes that are needed for your research.

In horticultural research you will probably use categories such as resistance to plant diseases, flower colour, size and shape, etc.

In social research categories such as age, gender, status, role or function in organizations, stated philosophy or ideology, may serve as starting points.

For study in-depth, you will need to select information-rich cases.

As your research progresses, you may discover new categories which would lead you to more sampling in that particular dimension.

Quota sampling

For quota sampling you will need to choose representative individuals from a specific subgroup.

For example, numbers in accordance with the demographic composition of the total population.

You can simplify the process by first preparing a matrix table that creates cells or strata.

You may wish to use gender, age, education, or any other attributes to create and label each stratum or cell in the table.

The research question and the focus of your study will determine which attributes you should select for your sample.

You should use a nonprobability method to fill the cells.

Each category in the overall sample must be filled using the same recruitment procedure for the resulting groups to be comparable.

Next, you need to determine the proportion of each attribute in the full study population.

Let’s say you want to study perceptions of violence among people over age 50.

You can, for example, group the people in your sample as younger than 30; 30 to 39; 40 to 49; 50 to 59 and 60 and older.

You will need to determine the proportion of people in each age group, although it will often be best to have the same number of individuals in each group.

The groups younger than 50 are included for comparison purposes.

Random sampling

In random sampling, each possible sample has an equal probability of being chosen as the sample.

The sample needs to be large enough to deliver findings that will apply to the entire target group with a reasonable accuracy.

This depends on the size of the population and the measure of heterogeneity.

The larger the population, the larger the sample drawn should be.

A random sample is mostly used when each element of the population has the same chance of being selected to be part of the sample.

This is sometimes called ‘simple random sampling’.

The procedure used to draw the sample needs to ensure that as little bias as possible is present.

Drawing names from a hat, putting the names on a dart board and throwing darts at them to choose sample items, the roll of a dice are examples of how a random sample can be composed.

Some dedicated computer software can also select a random sample from a list of names.

Reputational-case sampling

Reputational-case sampling is a variant of extreme-case and unique-case sampling.

Like in the case of the extreme-case and unique-case sampling, you will choose a sample recommended by an expert in the field.

For example, you might be looking for individuals who have particular expertise that is most likely to advance your research interests.

Snowball sampling

Snowball sampling is also called chain referral sampling or respondent-driven sampling.

Snowball sampling is an effective way to locate subjects with certain attributes or characteristics necessary in the study.

Snowball samples are particularly popular among researchers who are interested in studying various classes of deviance.

Sensitive topics are often studied, and the sample often come from difficult-to-reach populations.

In snowball sampling, you would identify a small number of individuals who have the characteristics in which you are interested.

These people are then asked to identify and put you in touch with others who also have the wonted characteristics.

For example, if you are doing research on people who suffer from arachnophobia,  you need to find only one person who is intensely scared of spiders and would be willing to put you in touch with others who suffer from arachnophobia to get the snowball rolling.

It so happens that this sampling method is especially useful for sampling a population where access and obtaining co-operation are difficult.

The basic procedure of snowball sampling begins by identifying one or more people with relevant characteristics and then interviewing them or asking them to complete a questionnaire.

After interviewing the subjects or after they have completed a questionnaire, you would ask them for the names of other people who possess the same attributes as them.

In this manner, a chain of subjects driven by the referral of one respondent to another is formed.

Stage sampling

Stage sampling is an extension of cluster sampling.

It involves selecting the sample in stages, i.e. taking samples of samples.

Doing research on the value systems of universities, you can first select a cluster of universities as your sample, then certain faculties in each university as a sub-sample, then a number of students from each faculty as your sub-sub sample.


A probability sample is a random sample for which the measure of probable accuracy is calculated.

In purposive sampling you would collect data based on the aim of your research. The characteristics of the population from which you will draw a sample will be taken into consideration.

A quota sample is drawn from a population that represents the profile of your research target. The profile is based on your research question and the focus of your study.

In random sampling each possible sample has an equal chance of being chosen for your research.

In reputational-case sampling you will choose a sample recommended by an expert in the field.

In snowball sampling you would identify a small sample or perhaps just one individual who meets the requirements for your research. This individual or small group can recommend other individuals or groups that meet your requirements.

Stage sampling is the selection of a sample in stages, that is taking samples from samples.

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ARTICLE 35: Research Methods for Ph. D. Studies: Sampling Part 3 of 6

Written by Dr. Hannes Nel


I will discuss event sampling, extreme-case sampling, matched sampling, multi-phase sampling, non-probability sampling and opportunistic sampling in this article.

Event sampling

Of all the types of sampling, event sampling is the most a research method rather than a data collection method.

You would typically select a small group of individuals to investigate.

It requires counting the number of times that an occurrence is observed or a statement, word, etc. is found in a data source, for example a document.

Counting words, statements, etc. in a document is easy if it is available in electronic format.

An example of this is to count the number of times that the words “he” and “she” respectively are used in a book to determine if there is gender discrimination against one or the other.

You can use your laptop computer to count the words for you.

Any word processor on computer has the facility to count words.

When your research requires you to observe certain repetitive actions, you will need to develop statements or criteria for the action.

This can be compared to the cricket scorebooks that some clubs still use, although computer programmes for even that have been developed.

The task becomes more difficult if it is not only necessary to count the number of times actions are repeated, but also the chronological order in which the actions took place.

An example of this is where you need to determine what kind of response a certain action invites, such as how people respond to a smile, compliment or insult.

Extreme-case sampling

Extreme-case sampling is used when information about unusual cases that may be particularly troublesome, or enlightening are sought.

Such a sample might represent the purest instance of a phenomenon that you wish to investigate.

Obviously, such cases are likely to be exceptions to the rule.

You might identify and investigate extreme cases to develop a richer, more in-depth understanding of a phenomenon.

You can also use extreme-case sampling to lend credibility to your research project.

Extreme-case sampling is often used in conjunction with other sampling methods.

In such events the extreme-case will be investigated in the context of the other sampling method.

The extreme-case can be used as an outlier, that is an observation that takes on an extremely high or extremely low value, thereby strengthening the norm.

An extreme-case can also be investigated on its own.

For example, where you do research on a specific individual who is extremely gifted, rich, strong, etc.

Matched sampling

In matched sampling one member of a control group is matched to a member of an experimental group.

Matching is done randomly, but the independent variables that are considered important for the research are taken into consideration.

Pairs of participants are matched in terms of the independent variables.

For example, age, gender, academic qualifications, etc. and only then assigned to the control or experimental group.

Matching pairs are used mostly when experimental research methods are used.

This implies that it is more valuable to quantitative research than to qualitative research.

However, experimental research methods can be used with qualitative research methodology.

Therefore, matching pairs can also be used to compose samples for such research.

Multi-phase sampling

Multi-phase sampling is a form of stage sampling.

The difference between multi-phase sampling and stage sampling is this:

In the case of stage sampling the criteria on each level are pretty much the same for each level of sampling in a particular region or field.

You will take samples of samples.

For example, your initial sample can be 1000 students randomly selected at a university.

Your next level sample can be 500 students who enrolled for psychology.

Your third level can be the 100 students who achieve the highest marks in the final exam.

In multi-phase sampling the criteria change on each ensuing level.

For example, the first stage criterion might be region.

The second stage criterion can be students who excel in numeracy skills.

The third stage criterion can be students with blue eyes.

Non-probability sampling

Convenience sampling, quota sampling, dimensional sampling, purposive sampling and snowball sampling can all be conducted in a non-probability sampling manner.

In non-probability sampling you, as the researcher, select the sample based on your own subjective judgement.

Non-probability infers that randomness ins not required, and the accuracy of the research findings need not be accurate to a good measure of probability, for example 90%.

In non-probability sampling, you will do research on a specific group to obtain knowledge about the group.

You will not claim general applicability of your findings.

The group, therefore, is important rather than any wider population.

This is often the case with small-scale research.

For example, just two or three faculties in a university,

Or just the lecturers in a small number of faculties.

Small-scale research often uses non-probability samples.

Meaning that not all available individuals have an equal chance of being selected for the sample.

The more expertise you have of the field of study, the better will you be able to select your sample.

Such samples are far less complicated to set up than a randomly selected sample.

However, keep in mind that they probably do not represent a substantive population.

Then again, they are less expensive than random samples and can achieve the purpose of the research if the general applicability of the findings is not important.

Small-scale research can also be used to pilot data collection tools, such as questionnaires, before they are used in actual research.

Opportunistic sampling

Opportunistic sampling means taking advantage of unanticipated events, leads, ideas, hints, and issues.

Accidental sampling is similar, but slightly different, as we saw in my previous post on convenience sampling.

The sample is composed of items upon which you stumbled by chance.

People who are available at the time of your study can be included in the sample.

They will need to provide valuable information and they must fit the criteria that you need.

A friend, who unknowingly shares valuable information with you at a party can be included in your sample.

Opportunistic sampling can be risky because the data that you collect can often not be generalized.

It might, furthermore, be difficult to corroborate the information.

Your source might not be reputable and might not have any credentials or qualifications making him or her an authoritative or expert source.


Event sampling is where you would do research on one or a small number of people, occurrences, or phenomena.

Extreme-case sampling also focus on just one or a small number of people, occurrences, or phenomena. Only here you will look for an unusual case or cases.

In matched sampling members of a control group are compared in terms of independent variables to members of an experimental group.

In multi-phase sampling the sample is broken down in terms of different criteria for each phase or level.

Non-probability sampling is selecting a sample that does not necessarily represent the population.

The result of this would be that your research findings cannot be generalized.

Opportunistic sampling means using any data sources that you can find without deliberately selecting them.

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ARTICLE 34: Research Methods for Ph. D. Studies: Sampling Part 2 of 6

Written by Dr. Hannes Nel


I discussed an introduction to sampling in my previous post.

I also discussed the first two types of sampling, namely boosted sampling and case study sampling.

To refresh your memory, boosted sampling is a type of sampling where you take specific steps to ensure that individuals who might otherwise have been omitted from the sample, are included.

Case study sampling is an investigation into a specific instance or phenomenon in its real context.

I will discuss cluster sampling, convenience sampling, critical-case sampling and dimensional sampling in this post.

Cluster sampling

A large and widely dispersed population brings with it, administrative problems.

Let’s say you want to investigate the impact of video games on teenagers.

It would be difficult to select and investigate a sample of, say, 1000 teenagers across the entire country.

You can reduce the time and work by selecting a smaller geographical area within the country.

This might have a negative effect on the generalizability of your findings.

It depends on your purpose with your research if this would be a problem or not.

But at least your research topic will now be viable.

This is an example of cluster sampling.

You can go one or more steps further by subdividing the geographical region into towns and towns into suburbs.

This is called ‘multi-stage cluster sampling’.

Limitations can be:

  • Younger teenagers will probably not be affected the same as older ones.
  • Teenagers in rural areas will not be affected the same as children living in urban areas.
  • There are many other possible variables that can have an impact on the results of the research.
  • For example, single-parent versus both parents in the family, personality traits, home conditions, etc.

The obvious risk with cluster sampling is that you can build in bias by excluding some possibilities.

The type and purpose of the research will determine if cluster sampling should be used or not.

Convenience sampling

Convenience sampling means selecting inputs and data from whoever happens to be available.

It is, thus, a rather opportunistic type of sampling.

It can also be accidental in nature, although there is a subtle difference between convenience sampling and opportunistic or accidental sampling.

Convenience sampling means making use of sources of information that you know of and that are easy to obtain.

Opportunistic or accidental samples are sources of information that you are not aware of in advance and that ‘falls into your lap’.

Convenience sampling is not a scientific way in which to collect data or to compose a sample.

It involves choosing the nearest individuals to serve as respondents.

It also means continuing the process until the required sample size has been reached.

Captive audiences such as students in a class are sometimes asked to discuss issues or complete questionnaires.

Convenience sampling is unlikely to deliver generally applicable findings.

Therefore, you will only use convenience sampling if achieving generalizable findings is not important to you and your research.

You must acknowledge that your findings are not applicable to a larger population than the small sample that you selected for your research.

It is even possible that your findings apply to your sample only.

Critical-case sampling

Critical-case sampling is sampling where it is important to obtain maximum applicability.

If the information and findings hold true for a critical case, it is likely to hold true for other cases and communities as well.

Critical cases are those that are likely to yield the most or most important information that will prove a hypothesis or solve a problem and that will have the greatest impact on the development of knowledge.

When selecting a critical-case sample, you would be looking for a ‘decisive’ case that would help you decide about which of several different explanations for a phenomenon, event or behavior is most plausible or most widely regarded as representing the general profile of a community.

For example, if one or a small number of people living in a jungle somewhere proves to have an immunity to the venom of a snake that can be found in that part of the world, one can probably deduce that there are people who have developed an immunity to the venom of that snake.

It does not necessarily mean that the tribe from which the sample was taken is immune to the venom of the snake.

This is a rather dramatic example and one will probably need to do more research and tests before you can claim with certainty that all people belonging to that tribe are immune to the venom.

It would also be interesting to do additional research on why the people are immune to the venom.

But please keep in mind that this example is a figment of my imagination – I don’t think there is such a tribe or snake.

Critical-case sampling is an efficient way in which to conduct research.

It need not be expensive because only one or a few items should be enough to prove your hypothesis or solve your research problem statement or question.

Dimensional sampling

Dimensional sampling is a refinement of quota sampling.

It can be used to reduce the sample size.

Dimensional sampling means selecting participants in the sample group in terms of a combination of criteria that you feel they should meet.

For example, when doing research on the correlation between perseverance and cognitive thinking ability, you can draw up a table with criteria for perseverance at the top (as column headings) and cognitive thinking ability down the side (as row headings).

A table like the one in the example can easily be converted into graphs of different types.


Cluster sampling is mostly used where the population is widely dispersed.

You will need to choose a cluster in the population that is representative of the population and that can realistically be investigated.

Convenience sampling means accepting any relevant data sources that you can easily find for your sample.

Convenience samples can be found by accident or coincidently.

Critical-case sampling focuses strongly on the purpose of your research.

The sample will often be too small to deliver generalizable findings.

Dimensional sampling is sampling where different characteristics of the sample are compared in the hope that the intersections of comparative data will deliver significant findings.

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ARTICLE 33: Research Methods for Ph. D. and Master’s Degree Studies: Sampling Part 1 of 6

Written by Dr. J.P. Nel


I will discuss sampling in this and the five postings following on this one.

How large should your sample be if you were to do research on factors that make some people more susceptible to the COVID-19 virus than others?

Would you even use a sample, or would that be irresponsible for a topic that threatens human life as we know it?

Should one entrust an individual with such a critically important issue, or would it be something that should be done by a large team of researchers in as many countries as possible?

The three questions that I just asked should already show that sampling is not suitable for all research topics.

It is valuable for academic purposes and can be used if the main purpose is to broaden our knowledge.

As we all know, sampling is used to study the social and medical impact of the COVID-19 virus.

We also know that the findings of such research are sometimes questioned.

And we know that findings are sometimes proven wrong.

Sampling is largely about data collection.

However, it also includes the analysis of data.

That is why I am discussing it as a research method rather than a data collection method.

Obviously, sampling cannot be a stand-alone research method.

It must always be linked to other research methods.

The type and size of sample that you choose for your research can determine if your research will succeed or not.

You should choose the size and type of sample after you have chosen the target group for your research.

In quantitative research, you might need to calculate the size of the sample based on what percentage probability of accuracy you will need.

Dedicated computer programmes can often work out the sample size for you.

In qualitative research factors like expense, time available and where your target group is will determine the size of your sample.

Where the target group is, impacts on distance and accessibility, which will determine what size sample you will be able to reach and deal with.

The data collection method or methods that you will use will also have an impact on the size of your sample.

Although a sample should be large enough to provide valid and generally applicable information for a community, it should be small enough for you to manage.

Sampling in qualitative studies focuses on the quality of the information collected and not on the number of participants.

A sample of ten to twelve people in a community of 100 people is often acceptable.

Researchers in natural sciences will probably frown upon such a small sample or the unscientific way the sample size is decided upon.

Keep in mind that the response rate to questionnaires is often low.

You will do well if you achieve a 10% or more response rate.

If you need to receive at least 100 completed questionnaires back, you will need to send out at least 1,000 questionnaires.

To play safe I would send out double that number, i.e. 2,000 questionnaires.

Because if you don’t receive enough completed questionnaires you might need to send out more later.

You will not be able to receive as many responses through interviewing as you would through sending out questionnaires via the internet or the postal system.

Then again, you will not need to hold as many interviews as you would send out questionnaires.

Your sample must be representative of the population forming your target group.

Representativeness is determined by the size and composition of your sample.

A too-large sample is better than a too-small one.

You should, for example, not send your questionnaires only to people living in an old age home if you are doing research on people of all ages.

A random sample in terms of gender, age group, population group, intellectual capacity, interests and many more are often needed.

I will share 26 different types of sampling with you in this and five future articles.

Obviously, it would not be a good idea to discuss all of them in one post.

Therefore, I will spread them over six posts as shown on the slides.

Here, I will start with boosted sampling and case study sampling.

This article:

  1. Introduction to sampling.
  2. Boosted sampling.
  3. Case study sampling.

Article 2:

  • Cluster sampling.
  • Convenience sampling.
  • Critical case sampling.
  • Dimensional sampling.

Article 3:

  • Event sampling.
  • Extreme case sampling.
  • Matched sampling.
  • Multi-purpose sampling.
  • Non-probability sampling.
  • Opportunistic sampling.

Article 4:

  1. Probability sampling.
  2. Purposive sampling.
  3. Quota sampling.
  4. Random sampling.
  5. Reputational-case sampling.
  6. Snowball sampling.
  7. Stage sampling.

Article 5:

  • Stratified sampling.
  • Systematic sampling.
  • Theoretical sampling.
  • Time sampling.

Article 6:

  • Typical-case sampling.
  • Unique-case sampling.
  • Volunteer sampling.
  • Closing remarks.

Boosted sampling

Boosted sampling is a variant of purposive sampling.

It is a type of sampling where you take specific steps to ensure that certain individuals or types of individuals, who might otherwise have been omitted or underrepresented in the sample, are included.

People with special needs are often not included in samples because there are so few of them.

You can, for example, “boost” the sample by intentionally including people with special needs.

It is important to ensure that all relevant people are represented in the sample if you intend to do statistical analysis and quantitative research.

You might need the responses of specific people even if you are doing qualitative research.

Case study sampling

A case study is an investigation into a specific instance or phenomenon in its real context.

It is used to illustrate a general principle, pattern behavior, etc.

Case studies can establish cause and effect in a real context.

The value of a case study is largely dependent on the size and composition of the sample.

The advantage of a case study is that it serves as a small sample of a large whole, which makes it much more manageable.

This, of course, will only be so if the case being investigated is representative of the entire population being investigated.

It is important to allow case study events to speak for themselves rather than to depend too much on your interpretation, evaluation and judgement.

Data should be collected systematically and rigorously.

This means that you need to prepare well for case study research.

You should never manipulate a case because that will lead to false information and conclusions.

It is, however, not wrong to select a case as your sample to fit the purpose of your research.

For example, it would not be wrong to select tramps in an area frequented by them if you are doing research on factors that make people decide to turn their backs on society.


Sampling includes data collection and analysis.

It is always used in combination with other research methods.

The size of the sample will be statistically calculated in quantitative research.

Factors like time, funds, data collection methods, the purpose and topic of the research will decide the size of the sample in qualitative research.

The sample must be representative of the population for your research.

In boosted sampling you will take deliberate steps to ensure that certain elements in the population are included in the sample.

A case study sample will focus on a specific group or phenomenon.

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