Sampling

Opportunity sample

Recruit those people who are most convenient or most available

Random sample

Use a random technique to randomly select participants from the wider population

Stratified sampling (3)

Subgroups (strata) in a population are identified

Participants are obtained from each subgroup in proportion to their occurrence in the population

Selection from the subgroups is done using a random technique

Volunteer Sampling

Advertising and collecting volunteers

Systematic sampling

Use a predetermined system to select participants, such as selecting every nth person (where n=any number)

Opportunity sample: Advantages

Easiest method (time saved)

Opportunity sample: Disadvantages (2)

Biased sample (not all members have an equal chance of being selected) 

Unrepresentative 

Random sample: Advantages (2)

Unbiased - all members have equal chance of being selected

Generalisable

Random sample: Disadvantages (2)

Impractical

May be unrepresentative due to chance

Stratified sample: Advantageous (3)

Unbiased

Generalisable

Representative - most

Stratified sample: Disadvantage (2)

Knowledge of population characteristics required

Time consuming

Systematic: Advantage (2)

Unbiased 

Generalisable

Systematic: Disadvantages (2)

Periodic traits may occur (if so the sample is not random or representative)

May be unrepresentative due to chance

Volunteer: Advantages (2)

Easy

Less chance of ‘screw you’ phenomenon

Volunteer: Disadvantage (2)

Unrepresentative

Demand characteristics (volunteers eager to please)