Sampling definitions Flashcards
Census
A census observed or measures every member of a population
Population
Population is the whole set of items that are of interest
Sample
A sample is a selection of observations taken from a subset of the population which is used to find out the population as a whole
Advantage of a census
It should give a completely accurate result
Give 3 disadvantages of a census
- time consuming
- cannot he used when the testing process destroys the item
- hard to process large quantity of data
Give 3 advantages of a sample
- less time consuming and expensive than a census
- fewer people have to respond
- less data to process than in a census
Give 2 disadvantages of a sample
- the data may not be as accurate
- the sample may not be large enough to give information about small sub-groups if the population
Sampling units
Individual units of a population
Sampling frame
Sampling units of a population are individually named or numbered to form a list called a sampling frame
Simple random sampling
A simple random sample of size n is one where every sample of size n has an equal chance of being selected. Every unit is allocated a unique number and these numbers are chosen at random by either generating random numbers or lottery sampling.
Systematic sampling
In systematic sampling, the required elements are chosen at regular intervals from an ordered list. The first unit to be chosen should be chosen at random
Stratified sampling
In stratified sampling, the population is divided into mutually exclusive strata (males and females for example) and a random sample is taken from each.
The size of each strata in the sample is proportional to the whole population in that strata
Give 3 advantages of simple random
- free of bias
- easy and cheap to implement for small populations and small samples
- each sampling unit has a known and equal chance of selection
Give 2 disadvantages of simple random
- not suitable when the population size or the sample size is large as it it is potentially time consuming
- a sampling frame is needed
Give 2 advantages of systematic sampling
- simple and quick to use
- suitable for large samples and large populations
Give 2 disadvantages of systematic sampling
- a sampling frame is needed
- it can introduce bias if the sampling frame is not random
Give 2 advantages of stratified sampling
- sample accurately reflects the population structure
- guarantees proportional representation of groups within a population
Give 2 disadvantages of stratified sampling
- population must be clearly classified into distinct strata
- selection within each stratum suffers from the same disadvantages as simple random sampling
Quota sampling
In quota sampling, an interviewer or researcher selects a sample that reflects the characteristics of the whole population. The population is divided into groups according to a given characteristic. The size of each group determines the proportion of the sample that should have that characteristic.
Opportunity sampling
Opportunity sampling consists of 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
Give 4 advantages of quota sampling
- allows a small sample to still be representative of the population
- no sampling frame is required
- quick and easy
- allows for easy comparison between different groups within a population
Give 3 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 increase number of groups, which adds time and expense
Give 2 advantages of opportunity sampling
- easy to carry out
- inexpensive
Give 2 disadvantages of opportunity sampling
- unlikely to provide a representative sample
- highly dependent on individual researcher
Continuous variable
A variable that can take any value in a given range
Discrete variable
A variable that can take only specific values in a given range
