Populations and samples Flashcards
Population & sampling unit
Population - whole set of items of interest
SU - individual units of a population
Census
Observes every member of the population
Pros and cons of census
Pros:
- completely accurate result
Cons:
- expensive and time consuming
- cannot be used when testing process destroys item
- hard to process large quantities of data
Sample
Selection of observations taken from subset of population, used to find out info about whole population
Pros and cons of sampling
Pros:
- less time consuming and cheaper than census
- less data to process
- fewer people have to respond
Cons:
- data may not be accurate
- sample may not be large enough
Sampling frame
numbered / ordered list of sampling units
Simple random sampling
sampling where every unit has equal chance of being selected
Pros and cons of simple random sampling
Pros:
- No bias
- Easy and cheap
Cons:
- sampling frame needed
- not suitable for large sample size
- potentially expensive and time consuming
Systematic sampling
elements chosen sequentially from an ordered list
Pros and cons of systematic sampling
Pros:
- simple and quick
- suitable for large population
Cons:
- sampling frame needed
- could introduce bias
Stratified sampling
division of population into strata, and and random sampling from there
number in stratum = (stratum size / population size) x overall sample size
Pros and cons of stratified sampling
Pros:
- sample accurately reflects population
- guarantees proportional representation
Cons:
- time consuming to classify into strata
- sampling frame needed
Quota sampling
selecting a sample representative of population
Pros and cons of quota sampling
Pros:
- quick, easy, inexpensive
- representative of population
- no sampling frame needed
Cons:
- bias introduced
- time consuming to make more groups when increasing sample size
- time consuming to make groups
Opportunity sampling
taking sample of first available people at time of study
