Sampling types etc.

random sampling

• Ps are selected using a random number technique

• All members are identified (sampling frame) and then the individuals are selected by getting a computer to generate random numbers

Stratified sample

• Sub-groups (strata) within a population are identified (e.g. different age groups)

• Then a pre-determined number of Ps are selected from each strata in proportion to their representation in the target population

volunteer sample

• Ps are selected by asking for volunteers in an advertisement or local/national newspaper 

Opportunity sample

• Ps are selected by using those people who are most easily available 

• e.g. people walking around a town centre

Systematic sample

• Ps are selected by following a rule

• e.g. every 5th person who walks past you

Benefits of random sampling

• Representative

• Findings can be generalised

• Researchers preferences don’t influence who takes part

Disadvantages of random sampling 

• sometimes very difficult to get a sampling frame 

• takes effort and time 

• those who you ask may decline

• sample is not guaranteed to be representative 

How to take a stratified sample

Benefits of stratified sampling 

• they are substantially unbiased 

• more likely to be representative 

Disadvantages of stratified sampling

• you have to have a detailed knowledge of the characteristics of the population in order to study them

• very slow process

Advantages of systematic sampling

• it is largely unbiased

• useful in field experiments

Disadvantages of systematic sampling 

• it is not truly random 

• everyone in the population does not have an equal chance of appearing in the sample 

Advantages of opportunistic and volunteer samples

• you can easily target specific groups or types of people

• P dropout will be less as they want to participate

• ethical issues around consent are lowered

• quick and easy

Disadvantages of opportunistic and volunteer samples

• biased and unrepresentative 

• can’t generalise 

• not typical 

• greater demand characteristics?