Sampling Methods in Reseach: Types, Techniques, & Examples

  • Sampling is the process of selecting a representative group from the population under study.
  • The target population is the total group of individuals from which the sample might be drawn.
  • A sample is a group of people who take part in the investigation. The people who take part are referred to as “participants.”
  • Generalizability refers to the extent to which we can apply our research findings to the target population we are interested in. This can only occur if the sample of participants is representative of the population.
  • A biased sample is when certain groups are over or under-represented within the sample selected. For instance, if only males are selected, or if the advert for volunteers is put into the Guardian, only people who read the Guardian are selected. This limits how much the study’s findings can be generalized to the whole population.

The Purpose of Sampling

In psychological research, we are interested in learning about large groups of people who all have something in common. We call the group that we are interested in studying our “target population.”

In some types of research, the target population might be as broad as all humans. Still, in other types of research, the target population might be a smaller group, such as teenagers, preschool children, or people who misuse drugs.

Studying every person in a target population is more or less impossible. Hence, psychologists select a sample or sub-group of the population that is likely to be representative of the target population we are interested in.

This is important because we want to generalize from the sample to the target population. The more representative the sample, the more confident the researcher can be that the results can be generalized to the target population.

One of the problems that can occur when selecting a sample from a target population is sampling bias. Sampling bias refers to situations where the sample does not reflect the characteristics of the target population.

Many psychology studies have a biased sample because they have used an opportunity sample that comprises university students as their participants (e.g., Asch ).

OK, so you’ve thought up this brilliant psychological study and designed it perfectly. But who will you try it out on, and how will you select your participants?

There are various sampling methods. The one chosen will depend on a number of factors (such as time, money, etc.).

Probability and Non-Probability Samples

Random Sampling

Random sampling is a type of probability sampling where everyone in the entire target population has an equal chance of being selected.

This is similar to the national lottery. If the “population” is everyone who bought a lottery ticket, then everyone has an equal chance of winning the lottery (assuming they all have one ticket each).

Random samples require a way of naming or numbering the target population and then using some raffle method to choose those to make up the sample. Random samples are the best method of selecting your sample from the population of interest.

  • The advantages are that your sample should represent the target population and eliminate sampling bias.
  • The disadvantage is that it is very difficult to achieve (i.e., time, effort, and money).

Stratified Sampling

During stratified sampling, the researcher identifies the different types of people that make up the target population and works out the proportions needed for the sample to be representative.

A list is made of each variable (e.g., IQ, gender, etc.) that might have an effect on the research. For example, if we are interested in the money spent on books by undergraduates, then the main subject studied may be an important variable.

For example, students studying English Literature may spend more money on books than engineering students, so if we use a very large percentage of English students or engineering students, then our results will not be accurate.

We have to work out the relative percentage of each group at a university, e.g., Engineering 10%, Social Sciences 15%, English 20%, Sciences 25%, Languages 10%, Law 5%, and Medicine 15%. The sample must then contain all these groups in the same proportion as in the target population (university students).

  • The disadvantage of stratified sampling is that gathering such a sample would be extremely time-consuming and difficult to do. This method is rarely used in Psychology.
  • However, the advantage is that the sample should be highly representative of the target population and therefore we can generalize from the results obtained.

Opportunity Sampling

Uses people from the target population available at the time and willing to participate. It is based on convenience.

An opportunity sample is obtained by asking members of the population of interest if they would participate in your research. An example would be selecting a sample of students from those coming out of the library.

  • This is a quick and easy way of choosing participants (advantage)
  • It may not provide a representative sample and could be biased (disadvantage).

Systematic Sampling

Chooses subjects in a systematic (i.e., orderly/logical) way from the target population, like every nth participant on a list of names.

To take a systematic sample, you list all the members of the population and then decide upon a sample you would like. By dividing the number of people in the population by the number of people you want in your sample, you get a number we will call n.

If you take every nth name, you will get a systematic sample of the correct size. If, for example, you wanted to sample 150 children from a school of 1,500, you would take every 10th name.

  • The advantage of this method is that it should provide a representative sample.
  • The disadvantage is that it is very difficult to achieve (i.e., time, effort, and money).

Sample size

The number of participants needed depends on several factors; the size of the target population is important. If the target population is very large (e.g., all 4-6 yr olds in Britain), then you need a fairly large sample in order to be representative.

If the target population is much smaller, then the sample can be smaller but still be representative. There must be enough participants to make the sample representative of the target population.

Lastly, the sample must not be so large that the study takes too long or is too expensive!

Olivia Guy-Evans

BSc (Hons), Psychology, MSc, Psychology of Education

Associate Editor for Simply Psychology

Olivia Guy-Evans is a writer and associate editor for Simply Psychology. She has previously worked in healthcare and educational sectors.

Saul Mcleod, PhD

Educator, Researcher

BSc (Hons) Psychology, MRes, PhD, University of Manchester

Saul Mcleod, Ph.D., is a qualified psychology teacher with over 18 years experience of working in further and higher education.