Sample
A selection of observations taken from a subset of the population which is used to find out more information about the population as a whole.
Population
The set of all the items that are of interest.
Census
A census measures or observes every member of a population.
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 destroys the item
- hard to process large quantity of data
Advantages of a sample
- less time consuming and expensive than a census
- fewer people have to respond
- less data than a census
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 of the population
Individual units of a population are known as…
Sampling units
Often, sampling units of a population are individually numbered to form a list called a…
Sampling frame
What happens in lottery sampling?
The members of the sampling frame could be written on tickets and placed into a ‘hat’. The required number of tickets would then be drawn out.
Simple random sampling
Every member of the population has an equal probability of being selected for the sample.
Systematic sampling
The required elements are chosen at regular intervals from an ordered list, but the first one has to be chosen at random.
Stratified sampling
The population is divided into mutually exclusive strata. Proportional representation is used to make each subgroup proportional with the size of the subgroup.
Formula for number of people we should use from each stratum
Number in stratum/ number in population x overall sample size
Simple random sampling: random number generator
Number each member 1-100. Use a random number generator to generate x random numbers between 1-100. Select the members who correspond to the numbers.
Advantages of simple random sampling
- free of bias
- easy and cheap for small samples and populations
- each sampling unit has an equal chance of selection
Disadvantages of simple random sampling
- not suitable for large samples and populations
- sampling frame needed
Systematic sampling
The required elements are chosen at regular intervals from an ordered list, the first interval is chosen at random.
Advantages of systematic sampling
- simple and quick to use
- suitable for large samples and large populations
Disadvantage of systematic sampling
- a sampling frame is needed
- bias introduced if sampling frame is not random
Stratified sampling
The population is divided into mutually exclusive strata and a random sample is taken from each
- number sampled in a stratum= number in stratum/ number in population x overall sample size
Advantages of stratified sampling
- sample accurately reflects population structure
- proportional representation of group within population
Disadvantages of stratified sampling
- population must be clearly classified into distinct strata
- same disadvantages as simple random sampling within each stratum
Quota sampling
Similar to stratified, but the quota of items to be included from the different subgroups isn’t necessarily proportional. It also doesn’t require a random sample within the subgroup, so bias is extremely likely.
