population and sampling

what is a target population?

the entire set of people that psychologists want to research

what is a sample?

a smaller group drawn from the target population

what does it mean if a sample is representative?

it is an accurate reflection of the larger population

what does it mean if a sample is generalisable?

the researchers have the ability to draw conclusions from their research that apply to the larger population outside of research

what are the 5 main sampling methods?

• opportunity

• volunteer

• random

• systematic

• stratified

what is an opportunity sample?

when researchers decide to select anyone who happens to be willing and available to participate that is local

what is the strength of opportunity sampling?

it is convenient - less costly in terms of both time and money

what are the weaknesses of opportunity sampling?

• unrepresentative - the sample is drawn from a very specific area, so findings can’t be generalised

• researcher bias - researchers can chose who they ask to participate

what is volunteer sampling?

it involves participants selecting themselves to be part of the sample

• e.g. through adverts, or simply raising their hand when asked by the researchers

what is the strength of volunteer sampling?

easy to collect participants - requires minimal input from the researcher so is less time consuming

what are the weaknesses of volunteer sampling?

volunteer bias - asking for volunteers may attract a certain profile of people: ones that are helpful or keen

• this may then affect how far findings can be generalised

what is random sampling and how is it carried out?

all members of the target population have an equal chance of being selected

• first, an complete list of all members is obtained

• then, all of the names are assigned a number

• finally, the sample is generated through the use of a lottery method, e.g. a computer-based randomiser

what is the strength of random sampling?

no researcher bias - completely random so researchers can’t influence which participants are chosen

what are the weaknesses of random sampling?

• time consuming to set up and carry out - difficult to obtain a full list of target population

• sample may still be unrepresentative - just because it’s random doesn’t mean it’s an accurate reflection of target population

• people may refuse to take part even though they were chosen

what is systematic sampling and how is it carried out?

every n^th number of the target population is selected

• a sampling frame is produced - a list of people in the target population organised a certain way, e.g. alphabetical order

• then a sampling system is nominated, e.g. every 5th or 10th person

• researcher then works through frame until sample is complete

what are the strengths of systematic sampling?

• avoids researcher bias - once system for selection is established, researcher has no influence over who is chosen

• usually representative

what are the weaknesses of systematic sampling?

• people may refuse to take part even though they were chosen

• time consuming to set up and carry out - difficult to obtain a full list of target population

what is stratified sampling and how is it carried out?

the composition of the sample reflects the proportions of people in strata (sub-groups) within the target population

• identify the different strata that make up the population

• work out the proportions needed for the sample to be representative

• participants that make up each stratum are selected using random sampling

what are the strengths of stratified sampling?

• avoids researcher bias - once target population has been sub-divided into strata, the participants are randomly selected

• method produces a representative sample - designed to accurately reflect the composition of the population

what is the weakness of stratified sampling?

complete representation of the target population isn’t possible - the identified strata can’t reflect all the ways people are different