Statistics: Sampling Methods Flashcards
What is a census?
Collecting information on every member of a population, usually through a survey.
What are the advantages and disadvantages of a census?
Adv:
- Accurate representation of the whole population
- Unbiased
Dis:
- For large population, lots of time, effort and money to carry out
- Difficult to ensure every member surveyed
- Using census for testing items is impractical if every item would be used up or damaged in the process
What is simple random sampling? Explain how to carry it out
Where a random selection of people or items is chosen, where each person or item has an equal chance of being in the sample and each selection is independent of the others.
Assign each member of the population a unique number, and generate random numbers until the desired sample size is reached (repeating the generation if the same number appears twice)
What are the advantages and disadvantages of simple random sampling?
Adv:
- Every member of the population has an equal chance of being chosen, so it is unbiased.
- Easy and cheap for small populations
Dis:
- Inconvenient over large areas and populations
What is systematic sampling? Explain how to carry it out
Where every nth person or item in a population is selected.
Number each member in the population, divide population size by sample size to get interval, then starting at a random point, move along population at interval and select person or item at each point.
What are the advantages and disadvantages of systematic sampling?
Adv:
- Simple and quick
- Suitable for large populations and samples
Dis:
- If interval coincides with pattern in population, could be biased
What is stratified sampling? Explain how to carry it out
If the population is divided into groups, same proportion of each group is selected for sample as there is in the population.
Divide the size of the category by the size of the population and multiply by the sample size to get the number of people or items from that group that should be in the sample. Then use random or systematic sampling to select.
What are the advantages and disadvantages of stratified sampling?
Adv:
- If categories are mutually exclusive, should give representative sample
- Useful when results likely to vary depending on the category
Dis:
- Extra process to decide who will be surveyed can be expensive
What is cluster sampling? Explain how to carry it out
When the population can be divided into distinct groups, you can randomly select groups to survey.
Divide population into clusters covering whole population. Randomly select clusters to use in sample based on required sample size. Either use all members or randomly sample within each cluster.
What are the advantages and disadvantages of cluster sampling?
Adv:
- Can be more practical
Dis:
- Could be less representative
- Not always possible to separate population into natural clusters
What is convenience sampling?
The sample is chosen from a section of the population that is most convenient for the sampler.
What are the advantages and disadvantages of convenience sampling?
Adv:
- Easy and inexpensive
Dis:
- Not very representative
- Likely to be very biased
What is quota sampling?
Population split into groups and members of the population are selected until each quota is filled.
What are the advantages and disadvantages of quota sampling?
Adv:
- Easy for interviewer
- Non-response less of a problem
Dis:
- Can be biased by interviewer if selection process does not represent whole population
What is volunteer sampling?
People choose to be in the sample in response to advertisement (incentives like money can be used). Volunteers can then be randomly chosen or all members can be used.
What are the advantages and disadvantages of volunteer sampling?
Adv:
- Little time or effort to find sample members
- Volunteers more likely to respond
- Could be only way to get people to take part
Dis:
- Can be trends within respondents, leading to bias
