Population

A set of items that are of interest

Census

Measures every member of a population

Sampling unit

A selection of the population used to estimate information about the population as a whole

Sample

A selection of the population used to estimate information about the population as a whole

Simple random sampling

Every element in the set has an equal chance of being selected

Systematic sampling

Members are chosen at regular intervals from an ordered list

Stratified sampling

The population is divided into groups e.g. male and female. The proportions should reflect the proportions of the population as a whole

Advantages of simple random sampling

Free of bias

Easy and cheap

Equal chance of selection

Disadvantage of simple random sampling

Not suitable for a large population or sample size

Advantages of systematic sampling

Simple and quick to use

Suitable for large samples and populations

Disadvantages of systematic sampling

A sampling frame is needed

Possible bias because there is not an equal chance of selection

Advantages of stratified sampling

Accurately reflects the population

Guarantees proportional representation of groups

Disadvantages of stratified sampling

Can be time consuming

Quota sampling

Similar to stratified

A researcher will select a sample that reflects the characteristics of the group

Opportunity sampling

The sample is taken from people available at the time and who fit the criteria needed

Like random

Quantitative data

Numerical data (eg hight)

Discrete data

Data which can only take certain values (eg only an integer)

Qualitative data

Non numerical data (eg colour, or worded answers)

Continuous data

Data that can take any value (eg height, length)

How do you tell that there isn’t a skew

Mean = median = mode

How do you tell that there is a positive skew

Mean > median > mode

How do you tell that there is a negative skew

Mean < median < mode

What is variance

Standard deviation squared