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Sampling Psychology

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Sampling Psychology

In psychology, the population is also called the target population. The population is the group of people in which a researcher is interested.

The size of the population can vary widely. Some studies target the entire human population, while others are interested only in a smaller group, for example, people from specific age groups or certain professions, and so on.

Usually, researchers can't recruit the entire population to participate in their studies. Therefore, researchers instead select a small group within the population called the sample. This method in psychology is called sampling.

The sample drawn should represent the population in which the researchers are interested to make generalisations about the population. Ideally, researchers would like to select a sample with the greatest representativeness and minimal bias. Researchers can then generalise the results to the target population with greater confidence.

Sampling Psychology Sampling the population StudySmarter

Sampling the population, Katarina Gadže, StudySmarter Originals (Made with Canva images)

What are the types of sampling in psychology?

The following section is about the different sampling methods you need to know in A-level psychology. This section is about explaining, implementing, and evaluating each sampling technique.

Opportunity sample

An opportunity sample is a sample in which individuals are selected who are most available.

How do we obtain the opportunity sample?

We can obtain an opportunity sample by asking members of the population if they are interested and willing to participate in the study.

Evaluation

  • Evaluation is the quickest and most convenient way to obtain a sample because participants are recruited based on availability.
  • However, there may be unavoidable bias since the sample is essentially self-selected.
  • Opportunity sampling has been criticised for its lack of representativeness and poor generalisability of results to the target population.

Voluntary sample

A voluntary sample is a sample recruited by self-selection. In other words, participants self-select and contact the researcher.

How do we obtain the voluntary sample?

We obtain voluntary samples through word of mouth or advertising.

Evaluation

  • Volunteer sampling is a quick and inexpensive way to obtain many potential participants.
  • However, volunteer sampling also runs the risk of bias similar to opportunity sampling because the sample is essentially self-selected.
  • Bias can occur because certain types of people who agree to participate in the study may differ systematically from those who do not.

Sampling Psychology Volunteer Sampling StudySmarter

Volunteer sampling, Canva

Random sample

A random sample is one in which everyone in the target population has an equal chance of being selected.

How do we obtain the random sample?

One possible method for selecting a random sample is the lottery method. Researchers assign a number to each potential participant and then create a list of random numbers to select participants for the random sample.

Evaluation

  • Random sampling is arguably the best sampling method because it will likely provide the most representative sample and generalisable results.
  • In addition, random samples are free from bias on the researcher's part, as the researcher does not influence the selection of participants.
  • Nevertheless, even a random sample is biased if selected individuals refuse to participate.
  • Moreover, a random sample is difficult to achieve because the recruitment process takes a lot of time and resources.

Sampling Psychology Random Sampling StudySmarter

Random sampling, Canva

Systematic sample

A systematic sample refers to selecting participants according to a set of patterns (also known as a sampling frame).

How do we obtain the systematic sample?

To draw a systematic sample, it is possible to list all the population members and then determine the desired sample size. If you divide the number of people in the target population by the desired sample size, you will get a number we call n. If you select every nth name, you will get a systematic sample of the size you want. For example, if you're going to draw a sample of 100 students from a university with 1,000 students, n = 1000/100 = 10, you can take every tenth name.

Evaluation

  • Systematic sampling is an objective method that can greatly reduce researcher bias.
  • However, as with random sampling, systematic sampling runs the risk of bias if selected individuals refuse to participate.
  • In addition, systematic sampling requires a complete list of the population, which is difficult to obtain and time-consuming.

Stratified sample

A stratified sample is one in which researchers select participants according to their frequency in the target population. The researcher identifies the different groups that make up the target population and calculates the proportions needed to make the sample representative.

How do we obtain the stratified sample?

The identified groups are called strata (or subgroups), such as gender or age groups. Participants are randomly selected from each stratum in proportion to their occurrence in the population. Therefore, the sample should reflect the relative percentages of subgroups in the population.

Evaluation

  • Unlike opportunity sampling, this method is more likely to represent the underlying population, but it is constrained by our understanding of which subgroups are essential.
  • However, complete representation is impossible because stratification is not perfect. Researchers may not have perfect information about the composition of their target population.
  • Stratified sampling is time-consuming. As a result, we rarely see a stratified sampling in psychological research.

Sampling Psychology - Key takeaways

  • The population is the group of people in which a researcher is interested.
  • For practical reasons, researchers select a small group of participants from the population to participate in the study, called a sample.
  • Ideally, researchers aim for a sample with the greatest possible representativeness and minimal bias to generalise the results with confidence to the target population.
  • Each sampling method has its advantages and disadvantages. Researchers choose their preferred sampling method after weighing the representativeness of the sample and the economic cost.

Frequently Asked Questions about Sampling Psychology

Opportunity, voluntary, random, systematic, and stratified sample.

Since researchers can't recruit the entire population to participate in a study, they 

select a small group within the population called the sample. This process is called sampling.

A systematic sample is an example of sampling that refers to selecting participants according to a set of patterns (also known as a sampling frame).


To draw a systematic sample, it is possible to list all the population members and then determine the desired sample size. If you divide the number of people in the target population by the desired sample size, you will get a number we call n. If you select every nth name, you will get a systematic sample of the size you want. For example, if you're going to draw a sample of 100 students from a university with 1,000 students, n = 1000/100 = 10, you can take every tenth name.

Final Sampling Psychology Quiz

Question

What is a population?

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The population is the group of people in which a researcher is interested.

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What is a sample?

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A sample is the smaller group of participants selected from the target group to participate in the study.

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What is an ideal sample?

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Ideally, researchers aim for a sample with the greatest possible representativeness and minimal bias.

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What is an opportunity sample?

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An opportunity sample is a sample in which individuals are selected who are most available.

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What is a voluntary sample?


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A voluntary sample is a sample recruited by self-selection. In other words, participants self-select and contact the researcher.

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What is a random sample?

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A random sample is one in which everyone in the target population has an equal chance of being selected.

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What is a systematic sample?

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A systematic sample refers to selecting participants according to a set of patterns (also known as a sampling frame).

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What is a stratified sample?

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A stratified sample is one in which researchers select participants according to their frequency in the target population.

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What is the purpose of using sampling frames?

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The purpose of a sampling frame is to collect and organise all sampling units that the target population consists of and that you can draw a sample from. 

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What is a sampling unit?

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A sampling unit refers to an individual that is included in the sampling frame.

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What is a sampling frame?

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A sampling frame is a source (e.g. list) that includes all sampling units so all members of your target population. 

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What are the characteristics of a good sampling frame?

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  • A sample frame should be systematically organised, so all the sampling units and information about them can be easily found.
  • A sampling frame should include all sampling units of your target population and exclude any units that are not part of the target population.
  • A sampling unit shouldn't be repeated more than once in a sampling frame.
  • Including some characteristics, and contact information with sampling units can make sampling more efficient. 

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What is an example of a sampling frame when your target population is the population of the UK?

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For example, the data from a recent census, electoral register or telephone directory.

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What is an example of a sampling frame when the target population is students at a school?

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The sampling frame would be a list of all students attending the school.

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What are the challenges of using sampling frames in research?


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  • Sampling frames might be incomplete, not include everyone in the population of interest. 
  • It's also a problem if they include people that are outside of the population of interest or a sampling unit is included repeatedly in the sampling frame. 
  • Sampling frames that do not include sufficient information about the sampling units might result in inefficient sampling.

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Some people that are part of the sampling frame might refuse to take part in the research. When is it a problem for a study?

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If the people that agree and refuse to take part in research differ in a significant way and it affects the representativeness of the sample.

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How are sampling frames used in research?

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Sampling frames are used to draw the samples for research. Having a list of everyone in your target population allows you to draw a sample for your study using a sampling method of your choice.

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What are the types of sampling frames?


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Types of sampling frames include frame lists and area frames.

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What are list frames?

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Frame lists are sampling frames that list every single sampling unit, for example, every single individual in the target population.

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What are area frames?

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Area frames include land units (e.g. cities or villages) which you can draw samples from, area frames can have a form of e.g. a satellite image or a list of different areas. 

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What is the difference between a sampling frame and the sampling process?

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A sampling frame is the database of everyone in your target population. The sampling process can be used to select a sample so a smaller group from the population that you can collect data from. 

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What problems are associated with using a telephone directory as a sample frame for a population of a certain region?

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Not everyone in the population is on the electoral register. Similarly, not everyone whose data is on the telephone registry still lives in the place they might be registered in.

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What is a target population?

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A target population is a group of people you generalise research findings to.

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What is snowball sampling?

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Snowball sampling is a sampling type where the initially sought after individuals are used to recruit more participants suitable for the study. 

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Snowball sampling is a type of...

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non-probability sampling

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What is non-probability sampling?

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It is a method of sampling whereby the subjects are selected in a subjective, non-random nature. 

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Snowball sampling is usually used in...

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quantitative research 

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What is linear snowball sampling?

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Linear snowball sampling is when the research is determined by a singular, 'linear' sequence of referrals which is initiated by one single variable. 

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Why might linear snowball sampling be disadvantageous?

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Because it takes a long time to gather the data and there may be delays. 

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The type of snowball sampling where the researcher screens the variables before accepting a participant is known as...

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exponential discriminative snowball sampling. 

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What is an advantage of exponential non-discriminative snowball sampling?

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It is a faster method that linear snowball sampling. 

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Why might some individuals not want to be directly recruited for a study? 

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Because of societal stigma. 

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Why might snowball sampling be used in research about rare conditions?

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Because an individual who has a rare condition is more likely to know others with it. 

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Why might such a selective sampling process be an issue?

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Because it may cause sampling bias which decreases the validity of the results.

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What is sampling bias?

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It is when data for a study is collected in a such a way that individuals in the population have a lower or higher probability to be sampled. 

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What is the key advantage of Exponential discriminative snowball sampling?

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The key advantage is that it is a selective process which leads to the best fit of variables. 

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Snowball sampling acts as a kind of...

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referral system 

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Which of the three types of snowball sampling would be the most time consuming?

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Exponential discriminative snowball sampling. 

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