7.8 Sampling Flashcards
define bias
a systematic distortion
define ‘generalisation’
Applying the findings of a particular study to the population.
define ‘opportunity sample’
A sample of participants produced by selecting people who are most easily available at the time of the study.
define ‘population’
The group of people that the researcher is interested in. The group of people from whom a sample is drawn. The group of people about whom generalisations can be made.
define ‘random sample’
A sample of participants produced by using a random technique such that every member of the target population being tested has an equal chance of being selected.
define ‘sampling’
The method used to select participants, such as random, opportunity and volunteer sampling, or to sample behaviours in an observation such as event or time sampling.
define ‘stratified sample’
A sample of participants produced by identifying subgroups according to their frequency in the population. Participants are then selected randomly from the subgroups.
define ‘systematic sample’
A sample obtained by selecting every nth person (where n is any number). This can be a random sample if the person is selected using a random method; you then select every nth person after that.
define volunteer bias
a form of sampling bias (distortion) because volunteer participants have special characteristics, such as usually being more highly motivated than randomly selected participants
define ‘volunteer sample’
A sample of participants that relies solely on volunteers to make up the sample. Also called a self-selected sample.
describe the various types of sampling methods
small group of people selected from a population
opportunity sample - recruit those easily available
random sample - use a random technique, e.g. lottery method or random number generator
stratified sample - identify relevant subgroups, randomly select appropriate proportion from each subgroup
systematic sample - select every nth person
volunteer sample - people respond to an advertisement
evaluate sampling methods
opportunity sample - easy because they are there - but biased
random sample - unbiased, but takes time
stratified sample - proportional, representative and unbiased, but time consuming
systematic sample - unbiased, but not truly random
volunteer sample - variety of participants, but volunteer bias
what is the ideal to be achieved when sampling
a representative sample of the population
therefore generalisations can be made
how do you perform an opportunity sample?
recruit those people who are most convenient or most available, for example people walking by you in the street or students at your school
strength of opportunity sampling?
the easiest method because you just use the first suitable participants you can find, which means it takes less time to locate your sample than if using one of the other techniques
limitation of opportunity sampling?
inevitably biased because the sample is drawn from a small part of the population, for example if you selected your sample from people walking around the centre of town on a monday morning then it would be unlikely to include professional people (because they are at work) or people from rural areas
how do you perform a random sample
the lottery method
random number table
random number generators
strength of random sampling
unbiased; all members of the target population have an equal chance of selection
limitation of random sampling
need to have a list of all members of the population and then contact all of those selected, which may take some time
how do you perform a stratified sample
subgroups (or strata) within a population are identified
participants are obtained from each of the strata in proportion to their occurrence in the population
selection from strata is done using a random sampling technique
strengths of stratified sampling
likely to be more representative than other methods because there is a proportional and randomly selected representation of subgroups
limitations of stratified sampling?
very time consuming to identify subgroups, then randomly select participants and contact them
how do you perform a systematic sample
use a predetermined system to select participants such as selecting every 5th person i.e.
the numerical interval is applied consistently
strengths of systematic sampling
unbiased as participants are selected using an objective system
limitations of systematic sampling
not truly unbiased/random unless you select a number using a random method and start with this person, and then select every nth person
how do you perform a volunteer sample
advertise in a newspaper or on a noticeboard or on the internet
strengths of volunteer sampling
gives access to a variety of participants (e.g. all the people who read a particular newspaper) which may make the sample more representative or less biased
limitation of volunteer sampling
sample is biased in other ways because participants are likely to be more highly motivated and/or with extra time on their hands.
they might be more highly motivated to be helpful, or more broke and needing the money offered for participation
this results in a volunteer bias
