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
