Definition of population
The whole set of items that are of interest
Definition of census
Observes or measures every member of a population
Definition of sample
A selection of observations taken from a subset of the population. Used to find out more about the population as a whole.
Advantage of census
Should give a completely accurate result
Disadvantage of a census
Time consuming and expensive
Cannot be used when the testing process destroys the item
Hard to process large quantity of data
Advantage of sample
Less time consuming and expensive than a census.
Fewer people have to respond.
Less data to process than in a census.
Disadvantage of sample
Data may not be as accurate.
Sample may not be large enough to give information about small sub-groups of the population.
Random sampling methods
Simple random
Stratified
Systematic
Non-random sampling methods
Quota sampling
Opportunity (convenience) sampling
Method of simple random sampling
Define the population
List the sampling frame
Number the sampling frame (1-N)
Use the RanInt function on a calculator to randomly generate numbers.
Select the corresponding members for the sample.
Advantage of simple random
Free of bias
Easy and quick to implement for small populations and samples
Each sampling unit has a known and equal chance of being selected.
Disadvantage of simple random
Not suitable when population/sample size is large as it can be time consuming, disruptive and expensive.
A sampling frame is needed.
Systematic sampling method
Define the population.
List the full sampling frame.
Number each member of the sampling frame (1-N).
Calculate the sampling interval S= population size/sample size.
Choose a random starting point between 1 and S (use RanInt function on calculator).
Select that stating value then every nth value after that.
Advantages of systematic sampling
Simple and quick use.
Suitable for large samples and populations.
Disadvantages of systematic sampling
A sampling frame is needed.
It can introduce bias if the sampling frame is not random.
Stratified sampling method
Define the population.
Divide the population into mutually exclusive strata.
Find the size of each stratum and the total population size.
Decide on the total sample size to find.
For each stratum, decide on the number of items to sample using:
Stratum size/population size x total sample size.
Within each stratum use simple random sampling to select the individuals.
List and number sampling frame.
Randomly generate (number found before) numbers and select the corresponding individuals for the sample.
Advantages of stratified sampling
Sample accurately reflects the population’s structure.
Guarantees proportional representation of groups within a population.
Disadvantages of stratified sampling
Population must be clearly classified into distinct classes.
Selection within each stratum suffers from same disadvantages of simple random sampling.
Advantages of quota sampling
Allows a small sample to still be representative of the population.
No sampling frame required.
Quick, easy and inexpensive.
Allows for easy comparison between different groups within a population.
Disadvantages of quota sampling
Non-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.
Advantages of opportunity sampling
Easy to carry out
Inexpensive
Disadvantages of opportunity sampling
Unlikely to provide a representative sample.
Highly dependent on individual researcher.
