What is a census?
A census observed or measured 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.
Advantage of a census
It should give a complete accurate result.
Disadvantages of a census
- Time consuming and expensive.
- Cannot be used when the testing process destroys the item.
- Hard to process a large quantity of data.
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.
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.
What are sampling units?
- Individual units of a population.
- Often sampling units of a population are individually named or numbered to form a list called a sampling frame.
Describe 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.
Describe systematic sampling
The required elements are chosen at regular intervals from an ordered list.
Describe stratified sampling
The population of divided into mutually exclusive strata (eg. males and females) and a random sample is taken from each.
What are the three methods of random sampling?
Simple random sampling
Systematic random sampling
Stratified sampling
Advantages of simple random sampling
- 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.
Disadvantages of simple random sampling
- Not suitable when the population size or the sample size is large as it is potentially time consuming, disruptive and expensive.
- A sampling frame is needed.
Advantages of systematic sampling
- Simple and quick to use.
- Suitable for large samples and large populations.
Disadvantages of systematic sampling
- A sampling frame is needed.
- It can introduce bias if that sampling frame is not random.
Advantages of stratified sampling
- Sample accurately reflects the population structure
- Guarantees proportional representation of groups within a population.
Disadvantages of stratified sampling
- Population must be clearly classified into distinct data.
- Selection within each stratum suffers from the same disadvantages as simple random sampling (not suitable when the sample size is large, time consuming, disruptive and expensive)
What are the two types of non-random sampling?
Quota sampling
Opportunity sampling
Describe quota sampling
An interviewer or researcher selects a sample that reflect the characteristics of the whole population.
Describe 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.
Advantages of quota sampling
- Allows a small sample to still be representative of the whole population.
- No sampling frame required.
- Quick, easy and inexpensive.
- Allows for easy comparison between different groups within a population.
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, which adds time and expense.
- Non-responses are not recorded as such.
Advantages of opportunity sampling
- Easy to carry out.
- Inexpensive.
Disadvantages of opportunity sampling
- Unlikely to provide a representative sample.
- Highly dependent on individual researcher.
