Sampling Psychology

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.

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

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

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.

Random sample

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.

Systematic sample

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

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.