Types of Sampling

What is a population?

• Whole set of things which you are interested in

• e.g. teacher wanted to know how long year 11 spent revising population would be year 11

What is a sample?

selected part (subset) of a population

e.g. certain number of year 11 not the whole year

What is a Random Sample?

everyone in the population has an equal chance of being selected

Biased Sample?

sample is not random

Census?

data is collected from every member of a population

advantage of a population?

• provides accurate results as everyone in the population has been used

• all opinions are included

disadvantage of a population?

• time consuming to collect data

• expensive because large numbers are involved

• large amounts of data to organise and analyse

advantage of sample?

• quicker to collect data

• cheaper (not a lot of work involved)

• less data to organise & analyse

disadvantage of sample?

• small sample size

• unreliable results (reliability improved by taking larger sample size)

• can introduce bias ( especially when sample is not random)

• might not be representative of population (only a selection of opinions could be accounted for or the members may all have the same opinions)

How do you increase reliability?

larger sample size will increase reliability

what is simple random sampling

when every member of the population has an equal chance of being selected

method for simple random sampling

uniquely number every member of a population randomly and select n different numbers using a random number generator

strength of simple random sampling

useful when you have a small population or want a small sample

limitation of simple random sampling?

• time-consuming if population or sample is large

• may not give sample that is representitive of the whole population

• cannot be used if not possible to number or list all members of the population

what is stratified sampling?

• population is divided into groups based on characteristics that may affect the investigation (age)

• random sample is taken from each group

• the proportion of group that is sampled is equal to the proportion of the population that belong to that group

method for stratified sampling?

• calculate the number of members sampled from each stratum

• use equation size of sample divided by size of population times the number of members in the group

• take a random sample from each group

strength of stratified sampling

• useful when there are very different groups of members within a population

• sample will be representative of the population structure

• members selected from each stratum are chosen randomly

limitation of stratified sampling

cannot be used if the population can not be split into groups of if the group overlap

what is cluster sampling?

• when the population naturally fall into groups (clusters) e.g. streets in a town

• a random number of clusters are selected and all members from within are used in the sample

method of cluster sampling?

identify the clusters, use all members in the selected clusters

strength of cluster sampling

• easy to complete

• cheeper and quicker than some other type of sampling

• when appropriate clusters are used the sample will be representative of the population

limitation of cluster sampling?

if appropriate clusters are not used sample may not be representative of the population

what is quota sampling?

when population is split into groups and members of population are selected until each quota is filled

method of quota sampling?

• calculate how many people you need from each group

• select members from each group until that quota is filled

• members do not have to be selected randomly e.g.people walking by

strength of quota sampling

• quick and inexpensive to complete

• useful when sampling frame is not available

limitations of quota sampling

• some members of the population might choose not to be included in the sample

• members may not be selected randomly