Simple random sampling

Advantages
Unbiased; easy and cheap to perform for small populations and samples; every member of the population has equal and known chance of being selected

Disadvantages
Not suitable when the population or sample size is large; a sampling frame is required

Systematic sampling

Advantages
Simple and quick to perform, even for large samples and populations

Disadvantages
A sampling frame is needed, which may introduce bias if not random

Stratified sampling

Advantages
Sample reflects population structure and guarantees proportional representation of groups within a population

Disadvantages
Population must be able to be distinctly classified into groups; selection within each group suffers from the same disadvantages as simple random sampling

Quota sampling

Advantages
Allows a small sample to still be representative of a poulation; no sampling frame required; quick, easy and cheap to perform; allows for comparison between different groups within a population

Disadvantages
Could be biased due to non-random nature; population must be divided into groups; increase scope of study increases number of groups; non-responders not recorded as such

Opportunistic sampling

Advantages
Easy to perform; cheap

Disadvantages
Sample highly like to be biased and unrepresentative of the population