Sampling

What is a population?

A group of people who are the focus of the researcher’s interest, from which a smaller sample is drawn.

What is a sample?

A group of people who take part in a research investigation.

Where is the sample drawn from?

A (target) population and is presumed to be representative of that population.

What is the difference between population and sample?

A population is the entire group that the researcher wants to draw conclusions about. A sample is the specific group that the researcher will collect data from.

What are the different types of sampling?

Random, systematic, stratified, opportunity and volunteer.

What is meant by random sampling?

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

How is random sampling carried out?

1. A complete list of all members of the target population is obtained.

2. All of the names on the list are assigned a number.

3. The sample is generated through the use of some lottery methods (this could be a computer-based randomiser or even picking names from a hat)

What are the advantages of random sampling?

• It is free from researcher bias.

Why is the lack of bias an advantage of random sampling?

The researcher has no influence over who is selected - CVs/EVs are controlled, enhancing internal validity.

What are the disadvantages of random sampling?

• It can be unrepresentative.

• Time-consuming.

Why might the sample be unrepresentative in random sampling?

The chosen sample might not represent the population very well because of random fluctuations.

Why is random sampling being consuming a disadvantage?

A complete list of the target population is hard to get. Some participants may also refuse to take part.

What is meant by systematic sampling?

Where every nth member of the target population is selected.

How is systematic sampling carried out?

1. A sampling frame is produced - a a list of people in the target population, organised into, for instance, alphabetical order.

2. A sampling system is nominated (every 3rd, 5th person, ect.) or this interval may be determined randomly to reduce bias.

3. The researcher then works through the sampling frame until the sample is complete.

What are the advantages of systematic sampling?

• Avoids researcher bias.

• Fairly representative.

How is the lack of researcher bias an advantage of systematic sampling?

Once the system for selection has been established, the researcher has no influence over who is chosen.

What are the disadvantages of systematic sampling?

A complete list of the population is required which takes time and effort.

What is meant by stratified sampling?

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

How is stratified sampling carried out?

1. The researcher identifies the different strata that make up the population.

2. The proportions needed for the sample need to be representative are worked out.

3. The participants that make up each stratum are selected using random sampling.

What are the advantages of stratified sampling?

• Avoids researcher bias

• A representative sample is produced.

How is the lack of researcher bias an advantage of stratified sampling?

Once the target population has been sub-divided into strata, the participants that make up the numbers are randomly selected, beyond their influence.

What are the disadvantages of stratified sampling?

• Stratification is not perfect.

Why is stratification not being perfect a disadvanatge of stratified sampling?

Strata cannot reflect all the ways in which people are different so complete representation is not possible.

What is meant by opportunity sampling?

When the researcher simply decides to select anyone who happens to be willing and available.

How is opportunity sampling carried out?

The researcher simply takes the chance to ask whoever is around at the time.

What are the advantages of opportunity sampling?

• Convenient

Why is convenience an advanatge of opportunity sampling?

It saves time and effort from the researcher and is cheap.

What are the disadvantages of opportunity sampling?

• Suffers from researcher bias.

• Sample is unrepresentative.

How is researcher bias a disadvantage of opportunity sampling?

The researcher has complete control over the selection of participants and may, for instance, avoid people they find intimidating.

How is the sample being unrepresentative a disadvantage of opportunity sampling?

It is drawn from a very specific area so findings cannot be generalised to the target population.

What is meant by volunteer sampling?

When participants select themselves to be part of the sample.

How is volunteer sampling carried out?

A researcher may place an advert in a newspaper of on a common room notice board. Alternatively, willing participants may simple raise their hands when the researcher asks.

What are the advantages of volunteer sampling?

• Easy.

• Less time-consuming

Why is volunteer sampling being easy and less time-consuming an advantage?

It requires minimal effort from the researcher as is less costly.

What are the disadvantages of volunteer sampling?

• Volunteer bias

What is volunteer bias?

Asking for volunteers may attract a certain ‘profile of a person‘ which may affect how findings are generalised.

What is meant by generalisation?

The extent to which findings and conclusions from a particular investigation can be broadly applied to the populations.

How is generalisation made possible?

If the sample of participants is representative of the population.