Types of Sampling Flashcards
What is random sampling?
A sampling method where each thing in our sampling frame has an equal chance of being chosen, in order to avoid bias
What is simple random sampling?
A method where every sample of the required size has the same probability of being selected.
How do you carry out simple random sampling?
In sampling frame each item has identifying number. Use random number generator, or βlottery samplingβ (names in a hat).
Advantages of simple random sampling
Bias free.
Easy and cheap to implement.
Each number has a known equal chance of being selected.
Disadvantages of simple random sampling
Not suitable when population size is large.
Sampling frame needed.
What is systematic sampling?
Required elements are chosen at regular intervals in ordered list
What are the three types of random sampling?
Simple random sampling, systematic sampling and stratified sampling
How do you carry out systematic sampling?
i.e. Take every kth elements where:
π=(πππ π ππ§π (π))/(π πππ π ππ§π (π) )
starting at random item between 1 and π.
Advantages of systematic sampling
Simple and quick to use.
Suitable for large samples/ populations.
Disadvantages of systematic sampling
Sampling frame again needed.
Can introduce bias if sampling frame not random.
What is stratified sampling?
Population divided into groups (strata) and a simple random sample carried out in each group.
How do you carry out stratified sampling?
Same proportion (π πππ π ππ§π (π))/(πππ π ππ§π (π) ) sampled from each strata.
Used when sample is large and population naturally divides into groups.
Advantages of stratified sampling
Reflects population structure.
Guarantees proportional representation of groups within population.
Disadvantages of stratified sampling
Population must be clearly classified into distinct strata.
Selection within each stratum suffers from same disadvantages as simple random sampling:
Not suitable when population size is large.
Sampling frame needed.
What are the two types of non-random sampling?
Quota sampling and Opportunity sampling
What is quota sampling?
A method where an interviewer or researcher selects a sample that reflects the characteristics of the whole population
What is opportunity sampling?
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 - aka convenience sampling
e.g. first 20 people you meet outside a supermarket on a Monday morning who are carrying shopping bags
Advantages of quota sampling
- Allows small sample to still be representative of population.
- No sampling frame required.
- Quick, easy, inexpensive.
- Allows for easy comparison between different groups in 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, adding time/expense.
- Non-responses are not recorded.
How do you carry out quota sampling?
The population is divided into groups according to a given characteristic (quota)
- Interviewer meets people and organises them into a quota
- This continues until all quotas have been filled
What happens if a person refuses to be interviewed in quota sampling?
You simply ignore them and move onto the next person
Advantages of opportunity / convenience sampling
- Easy to carry out.
- Inexpensive.
Disadvantages of opportunity / convenience sampling
- Unlikely to provide a representative sample.
- Highly dependent on individual researcher.
