Sampling techniques

• Random sample def

  Ps produced using random technique e.g. numbers in hat or random number generator (pseudorandom technique)
• Random sample strength 1

  unbiased - all members have equal chance of selection. But research may end up biased - not all Ps will agree to participate.
• RS strength 2

  Possible to choose specific subgroup in target pop first - easier to randomly select Ps
• RS weakness 1

  takes more time and effort than other techniques because need to obtain list of members of target pop then identify sample then contact people and ask to take part.
• RS weakness 2

  random samples often not random - not all Ps can be accessed or agree so it's more like volunteer
• Stratified sample def

  Ps selected from dif subgroups (strata) in target pop. Subgroups such as age groups are identified and represented as % of whole pop. e.g. 30% are 12 year olds so 30% in study should be 12. Random selection to identify.
• SS strength 1

  Most representative of all - subgroups are represented in proportion to numbers in target pop.
• SS strength 2

  Specific subgroups can be chosen according to the variables considered important by researcher - increases control over possible extraneous variables.
• SS weakness 1

  which subgroup to use may be biased thus reducing representativeness
• SS weakness 2

  lengthy process and Ps may not take part so technique not used much in psych research - more for opinion polls
• Volunteer sample def

  sample of Ps produced by asking people to take part eg ad in newspaper
• Volunteer sample strength 1

  convenient way to find willing Ps for time consuming studies - volunteers less likely to drop out
• VS strength 2

  specialised group of Ps (purposive sampling) e.g to study behaviour of med students put ad in med school not mall
• VS weakness 1

  biased as volunteers more likely highly motivated w more time than general pop (volunteer bias)
• VS weakness 2

  Volunteers more willing to be helpful so more prone to guessing aim of study and responding to 'demand characteristics'
• Opportunity Sample def

  sample produced by Ps most easily available at time of study e.g ask people walking by on street
• OS strength 1

  most convenient, little prep - less time to locate sample
• Os strength 2

  only technique available maybe. if whole target pop can't be listed
• OS weakness 1

  inevitably biased - sample drawn from specific part of target pop and not representative. e.g shoppers in mall on Monday morn are small part of pop and largely not people at work
• OS weakness 2

  Ps may refuse to take part so final sample has weaknesses of volunteer sample - more likely to respond to demand characteristics