A population by definition is a group of people with one or more characteristics in common. A population of people can also be defined as people who share a certain interest (eg. Cricket fans), have a common feature (eg. Left-handed people). The sample of that population is what’s recruited in a research or experiment.
The demographic taken should represent the population it's taken from so the findings of the research are representative and later generalizable. Target population of the study should also be recognized early on so that the sample the researcher chooses should be relevant and representative.
Important things to consider when sampling:
• Sample details such as age, ethnicity, gender. They are basic essentials that should always be considered.
• Sample details such as socio-economic standing, employment, education, occupation, geographical location.
• Sample size. (Should be balanced in terms of being representative)
• Small samples usually are less reliable and less representative and thus generally less valid to the clause of research.
An opportunity sample involves the researcher approaching people who are easy to find and easily available, such as students who are studying mathematics in the same university department. If a researcher is however interested in the ‘general demographic/public’ then it is possible for the researcher to approach people in places such as parks, student common rooms, shopping malls and etc.
Volunteer sampling (self-selecting sampling):
This usually revolves around researcher and experimenters advertising for participants. An advert could usually appear in a newspaper or on notice boards, online too. The people who reply are ‘self-selecting’ - that is they willingly volunteer themselves for the research. Sometimes volunteers are not given incentives at all, neither credentialled nor paid, often they are given a small amount of both or one of the other.
In this type of sampling technique each participant is given the equal chance of being selected from the target population. If the target population is ‘factory workers’ and there are 800 of them – the only way to actually randomly select the sample is to put all 800 names together and pick out the first 20, 30 names (depending on the required sample size of the study).