Standard Deviation
o = √Ex^2/n - mean^2
Census
Observing or measuring every member of the population
Population
the whole set of items that are of interest
Sample
A selection of observations taken from a subset of the population that are used to find out information about the population as a whole
Advantages of a census
Gives completely accurate results
Disadvantages of a census
- Time consuming and expensive
- Cannot be used when the testing process destroys the item
- Hard to process large quantity of data
Advantages of a sample
- Less time consuming and expensive than a census
- Fewer people have to respond
- Less data to process than a census
Disadvantages of a sample
- The data will not be as accurate
- The sample may not be large enough to give information
Sampling units
Individual units of a population
Sampling frame
When sampling units are individually named or numbered to form a list
Three methods of random sampling:
- simple random sampling
- systematic sampling
- stratified sampling
Simple random sampling
Every sample of size n has equal chance of being selected
Systematic sampling
The required elements are chosen at random intervals from an ordered list
Stratified sampling
The population is divided into mutually exclusive strata (e.g. males and females) and a random sample is taken from each
Two methods of non-random sampling:
- Quota sampling
- Opportunity sampling
Quota sampling
An interviewer or researcher selects a sample that reflects the characteristics of the whole population
Opportunity sampling
Taking the sample from people who are available at the time of the study and who fit the criteria they are looking for
