Sampling Flashcards
Population and sample
Population refers to the large group of individuals that a particular researcher is interested in studying. Target population because it is a subset of the general population.
Usually not all of target population is included for economic reasons so researchers use sample. Ideally sample that is drawn is representative of target population so that generalisation of findings becomes possible. However vast majority of samples contain some degree of bias. Samples are selected using a sampling technique that aims to produce a representative sample.
Random sample
All members of target population have a chance of being selected.
Obtain complete list of all members, all names assigned number, sample selected through lottery method. (Phone randomiser)
Systematic sample
Every Nth member of target population is selected, for example every third house on a street. A sampling frame is produced which is a list of people in the target population organised into, for instance, alphabetical order. A sampling system is nominated (every third, sixth etc.) May begin from a randomly determined start to reduce bias. The researcher then works through the sampling frame until the sample is complete.
Stratified sample
Sophisticated form of sampling in which the composition of the sample reflects the proportions of people in certain subgroups (strata) within the target population or the wider population.
To carry out a stratified sample the researcher first identifies the different strata that make up the population. Then the proportions needed for the sample to be representative are worked out. Finally the participants that make up each stratum are selected using random sampling.
Opportunity sample
Given that representative samples of the target population are so difficult to obtain, many researchers simply decide to select anyone who happened to be willing and available (an opportunity sample). The researcher simple takes the chance to ask whoever is around at the time of their study.
Volunteer sample
Involves participants selecting themselves to be part of the sample. May be advert.
Evaluation-random sample
Potentially unbiased. Means confounding or extraneous variables should be equally divided between the two groups, enhancing internal validity. However random sampling is difficult and time-consuming to conduct. A complete list of the target population may be extremely difficult to obtain.
Furthermore you may end up with a sample that is still unrepresentative- the laws of probability suggest that random sampling is likely to produce a more representative sample than, say, opportunity sampling.
Selected participants may refuse to take part. Applies to all methods.
Evaluation-systematic sample
Objective. Once the system for selection has been established the researcher has no influence over who is chosen.
As with random sampling this method is time consuming and participants may refuse to take part, resulting in volunteer sample.
Evaluation-stratified sample
Produces a representative sample because it is designed to accurately reflect the composition of the population. Means generalisation of findings becomes possible.
However stratification isn’t perfect. The identified strata cannot reflect all the ways people are different, so complete representation of a target population isn’t possible.
Evaluation-opportunity sample
Convenient. Much less time and money, list isn’t required
Opportunity samples suffer from two forms of bias. Sample is unrepresentative of the target population as it is drawn from a very specific area, so findings can’t be generalised to the target populations. Second, the researcher has complete control over the selection of participants and, for instance, may avoid people they don’t like the look of (researcher bias)
Evaluation-volunteer sample
Easy to collect, minimal input from researcher. Less time consuming. Researcher ends up with participants who are more engaged.
Volunteer bias is a problem. Asking for volunteers may attract a certain ‘profile’ of person, that is, one who is curious and more likely to try to please the researcher. (Not generalisable)
