sample

self selected (volunteer)

• people who sign up are usually more motivated (less likely to drop out).

• may not be representative of the target population.

opportunity (convenience)

• uses a pre-existing sample, easy to get readily available participants.

• may not be representative of the target population.

random

• all of the sample has an equal chance of selection (e.g. names out of a hat).

• minimises sampling bias.

purposive

• looks a a very specific set of traits.

• self selected, high sampling bias.

snowball

• specific target population, usually hard to reach (i.e. antarctica).

• one person of the population gather the rest of the participants.

stratified

• sample reflects the sub groups within the target population.

• takes into account the proportions of the population = more representative.