What is a Census?
A census measures or observes every member of a population.
what are the ADVANTAGES of a census?
Gives accurate results
What are the DISADVANTAGES of a Census?
- Time consuming and expensive
- Cannot be used in destructive testing
- Hard to process large quantity of data
What is a Population?
Whole set of items that are of interest
What is a sample?
A sample is a selection of observations taken from a subset of the population which is used to find out information about the population.
Generally the larger the sample the more accurate.
What are the ADVANTAGES of a Sample?
-Less time consuming and expensive than a census
- Fewer people have to respond
- Less data to process than a census
What are the DISADVANTAGES of a Sample?
- Data may not be as accurate
- Sample may not be as large enough to give information about sub groups of the population
What are sampling Units?
Individual units of a population
e.g taking a sample in the sixth form, I am a sampling unit.
What is a sampling Frame?
A sampling frame is where sampling units of a population are individually named or numbered to form a list.
What are the 3 types of Random Sampling?
- Simple Random Sampling
- Systematic Sampling
- Stratified Sampling
What is Simple Random Sampling?
A sampling Frame is required, each person or thing is allocated a number for random selection.
- either generating random numbers (computers, tables),or lottery sampling
- Lottery sampling - sampling frame or units can be written on tickets or put in a hat
What are the 2 types of methods for Simple Random Sampling?
Either generating random numbers (computers, tables), or lottery sampling
What are the ADVANTAGES of Simple Random Sampling?
- No bias
- Easy and cheap to implement for 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 sample size is large - Time consuming
- Sampling frame is needed.
What is Systematic Sampling?
The required elements are chosen at regular intervals from ordered list.
E.g Sample size of 20 from a populations of 100. k = 5 choose value from 1-5 and pick a number every 5th value
K = Population Size (N) / Sample Size (n)
What are the ADVANTAGES of Systematic Sampling?
- Simple and quick to use
- Suitable for large samples and large populations.
What are the DISADVANTAGES of Systematic Sampling?
- A sampling Frame is required
- if the sampling frame is not random there can be bias.
What is Stratified Sampling?
Population is divided into mutually exclusive strata and a random sample is taken from each.
Sample Size (n) / Population (N)
What are the ADVANTAGES of Stratified Sampling?
- Accurately reflects the population structure.
- Guarantees proportional representations of groups within a population.
What are the DISADVANTAGES of Stratified Sampling?
- Population must be clearly classified into a distinct area
- Selection of each stratum suffers from the same disadvantages as simple random sampling.
What are the 2 types Non - Random Sampling?
Quota Sampling
Opportunity sampling
what is Quota Sampling?
Interviewer or researcher selects a sample that reflects the characteristics of the whole population.
A quota of items/people in each group is set to reflect the groups proportion in the whole population - interview sets the actual sampling unit.
What are the ADVANTAGES of Quota Sampling?
No sampling frame required
- Not expensive
- Allos for easy comparison between groups
- Allows for a small sample to still be representative of the population.
What are the DISADVANTAGES of Quota Sampling?
- Can introduce Bias
- Population has to be divided into groups
- Non responses are not recorded
