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what is a population
the whole set of items that are of interest
what is a census
something that observes or measures every member of a population
what is a sample
a selection of observations taken from a subset of a population which is used to find out information about the population as a whole
advantages of a census
should give a completely accurate result
disadvantages of a census
time consuming and expensive
cannot be used when the testing process is destructive
hard to process large quantities of data
sample advantages
less time consuming and expensive
fewer people need to respond
less data to process
sample disadvantages
data may not be a s accurate
sample may not be large enough to give info about small sub groups of the populationj
individual units of a population are known as
sampling units - often individually named or numbered
what are the three methods of random sampling
simple random
systematic
stratified
in stratified sampling, the strata must be…
mutually exclusive
advantages of simple random sampling
free of bias
easy and cheap for small populations and samples
each sampling unit has a known and equal chance of selection
disadvantages of simple random sampling
not suitable when population/sample size is large as it is potentially time consuming, disruptive and expensive
sampling frame needed
systematic sampling advantages
simple and quick
suitable for large samples and populations
systematic sampling disadvantages
sampling frame needed
can introduce bias if sampling frame is not random
stratified sampling advantages
sample accurately reflects population structure
guarantees proportional representation of groups within a population
stratified sampling disadvantages
population must be clearly classified into distinct strata
selection within each stratum has the same disadvantages as simple random sampling
what is quota sampling
when the sample is chosen to reflect the characteristics of the whole population
opportunity sampling is
taking the sample from people who are available at the time the study is carried out and who fit the criteria you are looking for
quota sampling advantages
small sample can still be representative
no sampling frame required
quick easy and cheap
allows for easy comparison between different groups in a population
disadvantages of quota sampling
non random so can be biased
population must be divided into groups - costly/inaccurate
increasing scope of study increases number of groups - time and expense
non-responses not recorded as such
opportunity sampling advantages
easy and cheap
opportunity sampling disadvantages
unlikely to be representative
highly dependent on individual researcher
Note 
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