Introduction to Sampling Investigation & Design Produced by
Introduction to Sampling • In this topic we will consider how you decide who the people you study (participants) will be. • The first step is deciding who the target population will be. This is the particular group you are interested in studying. • For example, if we wanted to do some research on people studying A level Psychology, our target population will be students attending sixth forms and FE colleges.
â€˘ This number will be fairly too large, so the researcher will select a sample that would represent the target population so that the findings from your study can be generalised from that sample to the target population. â€˘ If the sample is not representative of the target population then the sample is said to be biased. â€˘ Sampling is the key consideration when considering external validity, in particular population validity.
Random sampling â€˘ Definition: a sample in which every member of the target population has an equal chance of being selected. â€˘ Method: Every member of the target population is identified and a random sampling technique is employed to select the sample. For example, names are drawn out of a hat or people are chosen from a numbered list using random number tables. â€˘ Population validity: this is a representative sample and has high population validity.
Opportunity sample â€˘ Definition: a sample that consists of those people available to the researcher. â€˘ Method: the researcher would approach people and ask them to take part in the research. Basically the researcher takes advantage of whoever happens to be available and is willing to take part in the research. â€˘ Population validity: high chance that the sample will be biased leading to low population validity.
Volunteer sample • Definition: a sample where the participants selfselect. That is they volunteer to take part in the research. • Method: the researcher would advertise their research and the people who respond would be the sample. • Population validity: researcher has found that a particular type of person is likely to volunteer for research, thus this type of sampling has a very high chance of bias and we cannot generalise to the public – therefore it has low population validity.