Stats Flashcards
(37 cards)
Define opportunity sampling
A sample is taken from the first n members of the population who are available and fit the criteria.
Define a simple random sample.
A simple random sample of size n is one where every sample of size n has an equal chance of being selected
Define a census.
When you collect information from every member of a population.
Advantages of a census.
- you get accurate information about pop, as every member has been surveyed
- true representation of population
- unbiased
Disadvantages of a census.
- takes lots of time and effort for large pops
- expensive
- can be difficult to make sure all members are surveyed
- if tested items used up or damaged in some way, census is impractical
Define sample units.
Individual members of the population
Define sampling frame.
A full list of all the sampling units. Used to represent population when selecting random sample.
Advantages of sampling.
- quicker and cheaper than census, and easier to get hold of required information
- only option when surveyed items are used up or damaged
Disadvantages of sampling.
- variability between samples, each possible sample will give different results, so could just happen to select one which doesn’t accurately reflect pop
- samples can easily be affected by sampling bias.
Advantage of simple random sampling.
- every member has equal chance of being selected, unbiased
Disadvantage of simple random sampling.
- can be inconvenient if pop spread over large area
Define systematic sampling.
Using a sampling frame, every nth member of the pop is sampled
Advantage of systematic sampling.
- used for quality control, should give unbiased sample.
Disadvantage of systematic sampling.
- regular interval could coincide with a pattern, which would make sample biased
Advantage of opportunity sampling.
- data can be gathered quickly and easily.
Disadvantage of opportunity sampling.
- isn’t random
- can be very biased, no attempt to obtain a representative sample.
Define stratified sampling.
- divide pop into categories
- calculate total pop
- calculate number needed for each category: size of category in pop/ total size of pop x total sample size
- select sample for each category at random.
Advantage of stratified sampling.
- guarantees proportional representation
Disadvantage of stratified sample
- not useful when aren’t any obvious categories
- can be expensive
Define quota sampling.
- divide pop into categories
- give each category a quota ( number of members to sample)
- collect data until quotas are met in all categories
Advantage of quota sampling.
- can be done when isn’t full list of pop
- every sample member responds as interviewer continues until all quotas are met
Disadvantage of quota sampling.
- easily biased by interviewer
Define cluster sampling.
- divide pop into clusters covering whole pop, where no member of pop belongs to multiple clusters
- randomly select clusters to use in sample
- either use all members of selected clusters or randomly sample within each cluster to form sample
Advantage of cluster sampling.
- more practical
- can incorporate other sampling methods at either stage, so quite adaptable
