sampling - populations and samples Flashcards
what is a population
a population is a large group of individuals that a particular research may be interested in studying, e.g. students attending colleges in the North West, children under six with autism, women in their thirties
this is called the target population
however for practical and economical reasons it is not possible to include all members of a target population in an investigation
why do researchers select samples rather than investigation the whole target population
it is not practical and economical to include all members of a target population in an investigation.
to solve this, researchers selects a smaller group known as the sample
what is an ideal sample
ideally, a sample that is drawn will be representative of the target population so that generalisation of findings becomes possible
This is very difficult to represent populations within a given sample.Therefore, a majority of sample contain some degree of bias
what is generalisation
generalisation is the extent of which findings and conclusions from a particular investigation can be broadly applied to the population
This is only made possible if the sample is representative of the population
how are samples selected
samples are selected using a sampling technique
the aim is to produce a representative sample
what is random sampling
a sophisticated form of sampling
all members of the of the target population have an equal chance of being selected
how do we select a random sample
Method:
- complete list of all members of the target population is obtained
- secondly, all of the names on the list are assigned a number
- sample is generated through the use of some lottery method ( computer - based randomiser/ picking numbers from a hat)
what is a systematic sample
in a systematic sample every nth numbs of the target population is selected
e.g. 3rd house on a street
how is a systematic sample selected
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 nth number). This interval may be determined randomly to reduce researcher bias
the researcher works through the sapling frame until the sample is complete
what is a stratified sample
a sophisticated form of sampling
it involves a composition of the sample which reflects the proportions of people in certain sub- groups (strata) within the target population/ wider population
how is a stratified sample selected
- the researcher first identifies the different strata that make up the population
- proportions needed for the sample to be representative are worked out
- participants that make up each stratum are selected using random sampling
what is a opportunity sample
given that representative samples of the target population are so difficult to obtain, many researchers simply decide to select anyone who happens to be willing and available
what is a volunteer sample
participants select themselves to be part of the sample (also referred to as self - selection)
in a volunteer sample, the researcher may place an advert in a newspaper or on a common room notice board
willing participates simply raise their hand when the researcher asks
what is a strength of random sampling
free from research bias
- this is because the researcher has no influence over who is selected and this prevents the form choosing people they may support their hypothesis
what are some limitations of random sampling
- difficult and time consuming to conduct
e. g. a complete list of a target population may be extremely difficult to obtain - the sample may be unrepresentative of the entire target population. However, due to the laws of probability, random sampling is still more Representative than opportunity sampling
- the participants may refuse to take part ( this may mean the researcher ends up with more of an volunteer sample