In Psychology, a sampling method is the way in which the researcher obtains the participants who will take part in their research. A sample is always drawn from the researcher’s ‘target population.’
In any study the target population is the group of individuals a researcher is interested in, for example, ‘babies in the Western world’, ‘people in the UK’, or ‘young people living in Bristol.’ At the end of the study, the researcher wants to make a statement about this population of people. This researcher obviously cannot study all the people in the target population (there are too many people in the target population). Instead, the researcher selects a smaller group called the sample.
Ideally, this sample will be representative of the whole target population so that generalisations about the population can be made. There are a number of sampling methods that researchers use in order to obtain a sample from the target population. It is important that these samples are representative of the target population, take a look at the strengths and weaknesses of each sampling method to assess whether or not the method would produce a representative sample.
Opportunity Sampling:Opportunity samples are made up of anyone who is willing and available to take part at the time of the study taking place. Opportunity samples are the most common sampling technique used in psychological research.
+ QUICK, CONVENIENT AND ECONOMICAL: For example, researchers simply need to approach individuals who are in the target population and ask them there and then for their participation in the study.This is positive as there is less planning and preparation required and so it leads to fewer delays in the research process and less money.
— CAN BE UNREPRESENTATIVE: For example, if a study in a city centre is conducted during work hours, this sample will not represent individuals who work, go to school, college etc…This is problematic because it means the sample cannot be generalised to the entire population meaning these samples can lack population validity.
Random Sample:A random sample gives every person from the target population an equal chance of being selected. Every name is given a number and a list of random numbers is then used to select the sample, either manually or using a computer.
+CAN BE REPRESENTATIVE: For example, every member of the target population has an equal chance of being selected as part of the sample making the sample more likely to be representative whereas an opportunity sample only considers who is available at the time.This is positive because random samples are more likely to have high population validity because everyone in the target population is part of the sampling process.
-CAN BE UNREPRESENTATIVE: For example, a disproportionate number of female college students might, by chance, be selected.This is a problem as the sample would be unrepresentative and so high population validity cannot be guaranteed.
Volunteer SampleThis consists of individuals who have volunteered to take part in research. Researchers may advertise for participants on university notice boards or in newspapers.
+ QUICK, CONVENIENT AND ECONOMICAL: For example, a researcher studying memory can advertise for participants and the participants should put themselves forward for the study.This is positive as there is less planning and preparation required and so it leads to fewer delays in the research process and less money spent.
— CAN BE UNREPRESENTATIVE: For example, it has been suggested that volunteers tend to be younger, more outgoing and more confident than non-volunteers.This is problematic as it means the findings of a study using this sampling method cannot be generalised to the entire target population and therefore the method may lack population validity.
Stratified SampleSubgroups (or strata) are identified in a population (e.g. boys and girls, age groups (10-12 years, 13-15 years). Participants are obtained from each of the strata in proportion to their occurrence of the population. Selection from the strata is done using a random technique.
+CAN BE REPRESENTATIVE: For example, the subgroups are selected in a proportionally and random way.This is positive because stratified samples are more likely to have high population validity because everyone in the target population has an equal opportunity of being selected and the sample is proportional to the target population therefore making the sample more representative of the target population.
-VERY TIME CONSUMING: For example, researchers have firstly got to identify the sub-groups, next carry out a random sample on these sub-groups and finally contact the participants.This is problematic because this process is very lengthy ion comparison to other sampling techniques and so may cause delays in research.
Systematic SampleUse a predetermined system to select every participant, such as selecting every 6th, 14th, 20th (or whatever) person from a phonebook. This numerical interval is applied consistently.
+ CAN BE REPRESENTATIVE: For example, all participants are selected using an unbiased and objective system which means that the sample should be representative of the target population.This is positive because systematic samples are more likely to have high population validity therefore making the sample more representative of the target population meaning that the findings from research can be generalised to the target population.
— CAN BE UNREPRESENTATIVE: As with a random sample, there is still the possibility that the sample will not be representative (for example, may by chance select participants with similar/the same characteristics/not represent participants with certain characteristics.This is problematic because the sample would be unrepresentative of the target population bringing about low population validity and making it difficult to generalise the findings from the sample to the target population.