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