What is a population?
The whole set of items that are of interest.
What does a census do?
It measures every member of a population.
What is a sample?
A selection of observations taken from a subset of the population which is used to find out information about the population as a whole.
What are sampling units?
Individual units of a population.
What is a sampling frame?
A list of individually named or numbered sampling units of a population.
What are the three methods of random sampling?
- Simple random sampling
- Systematic sampling
- Stratified sampling
What is a simple random sample?
One where every sample has an equal chance of being selected.
What is a systematic sample?
One where the required elements are chosen at regular intervals from an ordered list.
What is a stratified sample?
One where the population is ordered into mutually exclusive strata and a random sample is taken from each.
What are the 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
What are the disadvantages of simple random sampling?
- Not suitable when the population or sampling size is large
- A sampling frame is needed
What are the advantages of systematic sampling?
- Simple and quick
- Suitable for large samples and populations
What are the disadvantages of systematic sampling?
- A sampling frame is needed
- It can introduce bias if the sampling frame is not random
What are the advantages of stratified sampling?
- Sample accurately reflects the population structure
- Guarantees proportional representation of groups within a population
What are the 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
What are the two types of non-random sampling?
Quota sampling and opportunity sampling.
What is a quota sample?
One where an interviewer or researcher selects a random sample that reflects the characteristics of the whole population.
What is an opportunity sample?
One where a sample is taken from people who are available at the time the study is carried out and people who fit the criteria you are looking for.
What are the advantages of quota sampling?
- Allows a small sample to still be representative of the population
- No sampling frame needed
- Quick easy and inexpensive
- Allows for easy comparison between different groups within a population
What are the disadvantages of quota sampling?
- Can introduce bias
- Population must be divided into groups which can be costly or inaccurate
- Non-responses are recorded as such
What are the advantages of opportunity sampling?
- Easy to carry out
- Inexpensive
What are the disadvantages of opportunity sampling?
- Unlikely to provide a representative sample
- Highly dependent on individual researcher
What is a continuous variable?
One that can take any value in a given range.
What is a discrete variable?
One that can only take specific values in a given range.
