sampling Flashcards
what is a population?
the whole set of items that are of interest
what is a sample?
some subset of the population intended to represent the population
what is a sampling unit?
each individual thing in the population that can be sampled
what is a sampling frame?
when sampling units of a population are individually named or numbered to form a list
what is a census?
data collected from the entire population
what are the advantages of a cesus?
should give completely accurate result
what are the disadvantages of a census?
-time consuming and expensive
-can not be used when testing involves destruction
-large volume of data to process
what are the advantages of a smaple?
-cheaper
-quicker
-less data to process
what are the disadvantages of a sample?
-data may not be accurate
-data may not be large enough to represent small subgroups
what is a sampling error?
the difference between the actual value of the parameter and the value derived from a sample
what are parameters?
a calculation of a sample or population
what is bias?
a systematic error in the collection of the sample
what can lead to a biased sample?
-sample is not representative of the population
-leading questions
-the wrong person asking questions
-small sample size
what are the two types of sampling?
random and non-random
what is random sampling?
every item or person has an equal chance of being selected for the sample
what is non-random sampling?
sample selection is based on other factors than just random chance
what is simple random sampling?
every sample has an equal chance of being selected
how to carry out simple random sampling?
in a sampling frame, each item has an identifying number. use a random number generator to pick the sample
what are the advantages of simple random sampling?
-bias free
-easy and cheap to implement
-each number has a known equal chance of being selected
what are the disadvantages of simple random sampling?
-not suitable when the population size is large
-sampling frame needed
what is systematic sampling?
required elements are chosen at regular intervals in ordered list
take every kth element
where k=
pop size/ samp size
starting at random item between 1 and k
what are the advantages of systematic sampling?
-simple and quick to use
-suitable for large samples/populations
what are the disadvantages of systematic sampling?
-sampling frame needed
-can introduce bias is sampling frame is not random
what is stratified sampling?
-population divided into groups and a simple random sample carried out in each group
-same proportion samp size/pop size sampled from each strata
when is stratified sampling used?
when sample is large and population naturally divides into groups
what are the advantages of stratified sampling?
-reflects population structure
-guarantees proportional representation of groups within population
what are the disadvantages of stratified sampling?
-population must be clearly classified into distinct strata
-selection within each stratum suffers from same disadvantages as simple random sampling
what is opportunity/convenience sampling?
sample taken from people who are available at time of study who meet criteria
what are the advantages of opportunity/convenience sampling?
-easy to carry out
-inexpensive
what are the disadvantages of opportunity/convenience sampling?
-unlikely to provide a representative sample
-highly dependent on individual researcher
what is cluster sampling?
non-random stratified sampling
how to carry out cluster sampling?
-define each cluster (should be representative of population)
-collect random samples from each cluster
what are the advantages of cluster sampling?
-no sampling frame
-inexpensive
-choosing clusters instead of evaluating entire population
what are the disadvantages of cluster sampling?
unlikely to provide a representative sample because clusters tend to have similar characteristics resulting in overrepresentation within a cluster
what is quota sampling?
-population divided into groups according to characteristics
-a quota of items/people in each group is set to try and reflect the group’s proportion in the whole population
-interviewer selects the actual sampling units
what are the advantages of quota sampling?
-allows small sample to still be representative of population
-no sampling frame needed
-quick, easy inexpensive
-allows for easy comparison between different groups in population
what are the disadvantages of quota sampling?
-non-random sampling can introduce bias
-population must be divided into groups which can be costly or inaccurate
-increasing scope of study increases number of groups, adding time and expense
-non-responses are not recorded
what is the difference between cluster and quota?
in cluster you use simple random sampling in the groups, in quota you advertise and get volunteers until the ‘quota’ for each group is filled
