Populations, samples and sampling techniques

What are the 4 types of sampling

• opportunity sampling

• Random sampling

• Self-selected sampling

• Snowball sampling

What is opportunity sampling

• A sample produced by selecting people who are available at the time of study.

Give an example of opportunity sampling

• Asking people walking by you in the street or in your common room at school ie select those who are available.

Advantages of using opportunity sampling

• It is cost-effective, as no special equipment or large budget is needed to find participants.

• It allows the researcher to collect data in a short amount of time, making it useful when time or resources are limited.

• It can be practical for pilot studies, helping researchers test and refine their procedures before conducting larger studies.

Disadvantages of using opportunity sampling

• produces an unrepresentative sample, as those available at a certain time or place may share similar characteristics (e.g. mostly young students).

• It has a high risk of researcher bias, since the researcher decides who to approach or include, which can make the sample less objective.

• The lack of representativeness means findings cannot be easily generalised to the wider population.

What is random sampling

• A sample produced by using a random technique such that every member of the target population has an equal chance of being selected

Give an example of random sampling

• Placing all names of the target population in the hat and drawing out the required number or using a random number generator

  • Random number generators are pseudorandom techniques

What are the advantages of using random sampling?

• reducing researcher bias.

• It tends to produce a more representative sample, improving population validity.

• Because it is unbiased, results can be more easily generalised to the wider population.

• It is a simple and objective method if a complete list of the population is available.

What are the disadvantages of using random sampling?

• time-consuming and difficult to obtain a full list of everyone in the target population.

• Some selected participants may refuse to take part, leading to a smaller or biased sample.

• Random sampling doesn’t guarantee full representativeness — chance might still lead to an unbalanced sample.

• It can be impractical for large populations or field studies.

What is self-selected sampling/voluntary sampling?

• A sample of participants produced by asking for volunteers

Given an example of self-selected sampling

• Advertisements in the newspaper or on a noticeboard

What are the advantages of using a self selected sample?

• It is ethical, since participants give informed consent by choosing to take part.

• Participants are often motivated and committed, which can improve the quality and reliability of data.

• Useful when researchers need participants with specific characteristics or interests.

What are the disadvantages of using a self-selected sample?

• produce a biased and unrepresentative sample, as only certain types of people volunteer (e.g. those confident, interested, or with free time).

• Leads to volunteer bias, meaning results may not generalise to the wider population.

• Those who volunteer might try to please the researcher or behave differently, reducing validity.

• It might exclude groups who are less likely to respond to adverts (e.g. older adults or those without internet access).

What is snowball sampling?

• relies on referrals from initial participants to generate additional participants.

Given an example of snowball sampling

Current participants recruit further participants among people they know thus the sample group appears to grow like a snowball.

What are the advantages of using snowball sampling

• Useful for studying hard-to-reach or hidden populations, such as people with rare conditions or involved in sensitive topics.

• Convenient and cost-effective, as existing participants help recruit others.

• Enables research that would otherwise be impossible due to lack of access to certain groups.

• Can quickly build a large sample if participants are well-connected.

What are the disadvantages of using snowball sampling?

• It can create a biased and unrepresentative sample, since participants tend to recruit people similar to themselves.

• Confidentiality issues may arise if participants reveal others’ identities.

• The sample depends heavily on the social networks of initial participants, reducing generalisability.

• Lack of control over who joins the study can lead to ethical and methodological problems.

How many participants should be in questionnaires 

• it is relatively easy to distribute them to hundreds if not thousands of people 

How many participants should be in an experiment 

• Coolican 1996 suggests that as few as 25 is acceptable 

• It of often better to do a small sample then a big sample in experiments

• As big samples can obscure important individual differences

Why is using a small sample rather then a large sample better in experiments (example)

• consider a study looking at the effect of noise on memory

• It might be that noise has no effect on most peoples memory but does affect people with sensitive hearing

• If you have a very large sample, it may include enough people with sensitive hearing to affect the results

• So that it appears that all peoples memory is affected by noise