Stats; Data Collection Flashcards
Population
The whole set of items that are of interest.
Census
Observes or measures every member of a population
Sample
A selection of observations taken from a subset of the population, to find out about the population as a whole.
Sampling frame
A list of individually named or numbered sampling units.
Sampling units
Individual units of a population.
Census; advantages
Should give a completely accurate result.
Census; disadvantages
- Time consuming and expensive
- Cannot be used when testing process which destroys the item.
- Hard to process large quantity of data.
Sample; advantages
- Less time consuming and expensive than census
- Fewer people required
- Less data to process
Sample; disadvantages
- Data may not be as accurate
- Sample may be to small to give enough info about small sub-groups of the population.
Random sampling methods
simple random sampling, systematic, stratified
Random sampling
Every member of population has equal chance of being selected.
Representative of population.
Removes bias.
Simple random sampling
This sample of size ‘n’ is one where every sample of size ‘n’ has equal chance of being selected.
Simple random sampling; advantages
- Free of bias
- Easy and cheap to implement for small populations and samples.
- Each sampling unit has a known and equal chance of selection.
Simple random sampling; disadvantages
- Not suitable for large population or samples sizes.
- Needs a sampling frame.
Stratified sampling
The population is divided into mutually exclusive strata (e.g. males and females) and a random sample is taken from each.
Stratified sampling; advantages
- Sample accurately reflects population structure.
- Guarantees proportional representation of groups within population.
Stratified sampling; disadvantages
- Population must be clearly classified into distinct strata.
- Selection in each stratum has same disadvantages as simple random sampling.
Systematic sampling
The required elements are chosen at regular intervals from an ordered list.
Systematic sampling; advantages
- Simple and quick
- Suitable for large samples and populations.
Systematic sampling; disadvantages
- Needs sampling frame
- Can introduce bias if the sampling frame is not random.
Non-random sampling methods
quota sampling, opportunity sampling
Quota sampling
An interviewer/researcher selects a sample that reflects the characteristics of the whole population.
Quota sampling; advantages
- 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.
Quota sampling; disadvantages
- Non-random sampling can introduce bias.
- Population must be divided into groups, costly or inaccurate.
- Increasing scope of study increases number of groups, adds time and expense.
- Non-responses are not recorded as such.
Opportunity (convenience) sampling
Consists of 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.
Opportunity sampling; advantages
- Easy to carry out
- Inexpensive
Opportunity sampling; disadvantages
- Unlikely to provide a representative sample.
- Highly dependent on individual researcher.
