1. The Scientific Method in Psychology
Sampling of participants

2. The Scientific Method in Psychology
Sample and population
Participants are the people who choose to take part in a study. They form the sample of participants whom the study will e conducted on. It is hoped that these people represent a wider, target population (TP).
The researcher must have a TP.

3. The Scientific Method in Psychology
Sample and population
The TP is the group of people a researcher studies in the same way in the hope that the findings can be generalised to and be representative of that TP.

4. The Scientific Method in Psychology
Sample and population
For example, if a psychologist wants to investigate memory in school children then the researcher’s TP might be children 5-8 years old.
There are different sampling techniques that psychologists can use to help recruit their participants from the TP.

5. The Scientific Method in Psychology
Opportunity sampling
Opportunity sampling is the sampling technique most used by psychology students. It consists of taking the sample from people who are available at the time the study is carried out and fit the criteria you are looking for. This may simply consist of choosing the first 20 students in your college canteen to fill in your questionnaire.

6. The Scientific Method in Psychology
Opportunity sampling strengths
It is a popular sampling technique as it is easy in terms of time and therefore money. For example the researcher may use friends, family or colleagues. It can also be seen as adequate when investigating processes which are thought to work in similar ways for most individuals such as memory processes.

7. The Scientific Method in Psychology
Opportunity sampling
Sometimes, particularly with natural experiments opportunity sampling has to be used as the researcher has no control over who is studied.
However, there are many weaknesses of opportunity sampling.

8. The Scientific Method in Psychology
Opportunity sampling weaknesses
Opportunity sampling can produce a biased sample as it is easy for the researcher to choose people from their own social and cultural group. This sample would therefore not be representative of your target population as your friends may have different qualities to people in general.

9. The Scientific Method in Psychology
Opportunity sampling weaknesses
A further problem with opportunity sampling is that participants may decline to take part and therefore the participants chosen may be an even more biased sample as those participants responding may be a particular type of person.

10. The Scientific Method in Psychology
Self selected sampling
Self selected sampling (or volunteer sampling) consists of participants becoming part of a study because they volunteer when asked or in response to an advert. This sampling technique is used in a number of the core studies.

11. The Scientific Method in Psychology
Self selected sampling strengths
This technique, like opportunity sampling, is useful as it is quick and relatively easy to do. It can also reach a wide variety of participants.
People are more likely to participate if they have already volunteered so the drop-out rate should be lower, making generalisations potentially stronger.

12. The Scientific Method in Psychology
Self selected sampling weaknesses
The type of participants who volunteer may not be representative of the target population for a number of reasons. For example, they be more obedient, more motivated to take part in studies and so on.

13. The Scientific Method in Psychology
Random Sampling
This is a sampling technique which is defined as a sample in which every member of the population has an equal chance of being chosen. This involves identifying everyone in the target population and then selecting the number of participants you need in a way that gives everyone in the population an equal chance of being picked.

14. The Scientific Method in Psychology
Random Sampling
For example, you could put all of the names of the students at your school in a hat and pick out however many you need.
Random sampling is the best technique for providing an unbiased representative sample of a target population.

15. The Scientific Method in Psychology
Random Sampling strengths
The researcher can generalise to the TP with more confidence. This is because the sample is more likely to be representative of the TP.

16. The Scientific Method in Psychology
Random Sampling weaknesses
Random sampling can be very time consuming and is often impossible to carry out, particularly when you have a large target population, of say all students. For example if you do not have the names of all the people in your target population you would struggle to conduct a random sample.

17. The Scientific Method in Psychology
Random Sampling weaknesses
If you ask people to volunteer for a study the sample is already not random as some people may be more or less likely to volunteer for things.
Similarly if you decided to put out an advert for participants it would be almost impossible to guarantee that every member of your target population has an equal chance of viewing the advert.

18. The Scientific Method in Psychology
Stratified Sampling
Stratified sampling involves classifying the population into categories and then choosing a sample which consists of participants from each category in the same proportions as they are in the population.

19. The Scientific Method in Psychology
Stratified Sampling
For example, if you wanted to carry out a stratified sample of students from a sixth form college you might decide that important variables are sex, 1st or 2nd years, age, have a part-time job and so on. You could then identify how many participants there are in each of these categories and choose the same proportion of participants in these categories for your study.

20. The Scientific Method in Psychology
Stratified Sampling strength
The strength of stratified sampling is therefore that your sample should be representative of the population.

21. The Scientific Method in Psychology
Stratified Sampling weaknesses
Stratified sampling can be very time consuming as the categories have to be identified and calculated.
As with random sampling, if you do not have details of all the people in your target population you would struggle to conduct a stratified sample.

22. The Scientific Method in Psychology
Sampling
There are many different ways of sampling participants, we have looked at just a few.
Watch the video for more information on sampling

23. The Scientific Method in Psychology
Sampling

24. The Scientific Method in Psychology
Sampling
Now take a look at the worksheet and try to match the information to the different sampling methods.