1. Sampling General population General population Target population    
General population
General population  
Target population
Target population 
Sample
Sample     

2. Sampling
Researchers aim to get a sample that is representative of the target population
Every type of person in the TP should be proportionally represented in the sample
NB: it is not necessary for the sample to be representative of the GP unless the researcher intends to generalise

3. Sampling Opportunity sampling Volunteer sampling
Use whoever is immediately available e.g. go out and accost passers-by
Volunteer sampling
Use whoever puts themselves forward e.g. advertise in the paper

4. Sampling Quota sampling Random sampling
Design a sample to reflect the TP and then recruit to order
Random sampling
Every member of the target population has an equal chance of being selected
E.g. selected by computer from the electoral roll

5. Sampling random random quota quota How difficult to obtain opportunity
volunteer
volunteer
How likely to be representative