Sampling Psychology

Sample in Psychology Definition

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 in making 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.

But what's the difference between a population and a sample? Aren't they the same thing? No, they are different! Remember, a population is an entire group that researchers are concluding. Differently, a sample is a specific group that the data is being collected.

Types of Sampling in Psychology

Two different types of sampling can be done within psychological research -- probability and non-probability.

Probability sampling means that every member of the population has a chance of being selected and is mainly used in quantitative research. What if you wanted to have results that represented the whole population? These desired results can be attained when using probability sampling. Probability allows researchers to make a strong inference about the whole group of interest.

In non-probability sampling, individuals are selected based on non-random criteria. Using non-probability sampling means that not all individuals have an equal chance of being included. Non-probability sampling is considered to be a cheaper option and much easier. While cheap and easy sounds great for researchers, there is a high sampling risk (meaning that inferences made about the population are weak from the samples taken, and conclusions will be ruled as limited).

Exploratory and qualitative research use non-probability sampling, where the aim is not to test a hypothesis about a broad population but instead to create an initial understanding of an under-researched population (usually also small).

Stratified Sampling in Psychology

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.

The identified groups are called strata (or subgroups), such as gender or age. 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.

• 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 Methods in Psychology

While stratified seems to be the go-to of sampling techniques, there are several different types depending on the research being conducted: opportunity, voluntary, random, and systematic.

Sampling Techniques Psychology: Opportunity Sampling

An opportunity sample is a sample that is recruited based on whoever is available.

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

• 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 to the target population.

Sampling Techniques Psychology: Voluntary Sampling

A voluntary sample is a sample recruited by self-selection. In other words, participants self-select and contact the researcher. We obtain voluntary samples through word of mouth or advertising.

• 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.

Voluntary sampling is when researchers seek volunteers to participate in studies. Volunteers can be engaged in person, on the internet, or through public postings. Researchers using voluntary sampling typically make little effort to control sample composition.

Fig. 1 Texting in your vote for the next big singing sensation is voluntary sampling.

Sampling Techniques Psychology: Random Sampling

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

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.

• 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 time and resources.

Sampling Techniques Psychology: Systematic Sampling

A systematic sample is selecting participants according to a set of patterns.

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.

• 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.

Sampling Techniques Psychology: Volunteer Sampling

Voluntary sampling is when researchers seek volunteers to participate in studies. Volunteers can be engaged in person, on the internet, or through public postings. Researchers using voluntary sampling typically make little effort to control sample composition.