research methods - sampling

What is a population ?

What is a population ?

a group of people who are the focus of the researchers’ interest from which a smaller sample is drawn

for example population could be university students - this is often called the target population because it is subset of the general population

what is a sample

this a group of people who take part in a research investigation

the sample is drawn from the target population and is presumed to be representative of that population i.e. it stands fairly for the population being studied

this is done because it is not practically and economically feasible to study a whole target population

why do a majority of samples have a degree of bias

it is very difficult to represent populations because in any given sample due to the diverse nature of populations i.e. people with different ages, genders interest and experiences

what is a sampling technique ?

the method used to select people from a population

what is bias

this is when certain groups are over or under represented within the sample selected.

i.e. to many younger people or to many people of one ethnic background in one sample

this limits the extent to which generalisation can be made to the target population

What is random sampling ?

this is a form of sampling in which all members of the target population have an equal chance of being selected.

How is a random sample selected

first researcher needs a list of all members in the target population

second, all names on the list are assigned a number

finally, the actual sample is selected through the use of some lottery method i.e. a random number generator / picking numbers from a hat

what is systematic sampling ?

this is when every nth number of the target population is selected e.g. every third house on the street/ every 5th pupil on the school register

how is a systematic sample selected ?

a sampling frame = produced - this is a list of people in a target population organised into for instance alphabetical order.

a sampling system is nominated every 3rd, 6th/ 8th person etc.

may begin from a randomly determined start to reduce bias

the researcher then works through the sampling frame until the sample is complete

What is stratified sampling ?

this is a firm of sampling in which the composition of the sample reflects the proportion of the people in certain subgroups (aka strata) within the target population or the wider population

how is a stratified sample collected ?

the researcher must first identify the different strata that make up the population.

then the proportions needed for the sample to be representative are worked out.

finally the participants that make up each stratum are selected using random sampling.

i.e. 40% of ppl living in Manchester support Man U

40% support Man city

5% support Leeds

15% support Bolton

in a stratified sample of 20 participants :

8 man city fans

8 man u fans

3 Bolton fan

1 Leeds fan

What is opportunity sample ?

this is when the researcher selects anyone who happens to be willing and available

how is an oppotunity sample selected ?

the researcher simply takes the chance to ask whoever is around at the time of their study , for example in the street

what is a volunteer sample?

this involves participants selecting themselves to be part of a sample - also referred to as self selection

how is a volunteer sample selected ?

researchers may place an advert in a newspaper or on a common room noticeboard. Alternatively willing participants may simply raise their hands when participants ask

What are the advantages of a random sample

they are potentially unbiased = this means that confounding/extraneous variables should be equally divided between the different groups enhancing internal validity

what are the disadvantages of random sampling ?

it can be difficult + time consuming - a complete list of the target population may be hard to obtain

may end up with an unrepresentative sample - however more likely to produce a representative sample than opportunity sample

selected participants may refuse to take part in the experiment - which means u end with something more like a volunteer sample

what are the advantages of a stratified sample ?

it produces a represent sample because it is designed to accurately reflect the composition of the population - generalisation of findings are possible

what are the disadvantages of a stratified sample ?

stratification is not perfect : the identified stratified cannot reflect all the ways that people are different , so complete representation is not possible

what are the advantages of systematic sample ?

this is an objective method - once the method of selection has been established the researcher has no influence over who is chosen

what are the disadvantages of systematic sampling ?

this method is time consuming and, in the end participants may refuse to take part resulting in a volunteer sample

what are the advantages of an opportunity sample ?

it is convenient

less costly in-terms of time and money that random sampling - because a list of members from the target population is not required

no need to split population into different strata’s

what are the disadvantages of an opportunity sample ?

sample = unrepresentative of the target population as it is drawn from a specific area i.e. on street or town

findings cannot be generalised to the target population

the researcher has complete control over the selection of participants may avoid people they do not like the look of (researcher bias)

what are the advantages of a volunteer sample ?

collecting a volunteer sample = easy

requires minimal input = less time consuming that other forms of sampling

researcher ends up with participants that are more engaged - more so than someone who they just stopped on the steet

what are the disadvantages of a volunteer sample ?

volunteer bias - problem

asking for volunteers may attract a certain type of profile of person - one who is more curious and likely to please the researcher

this could limit how far the findings can be generalised