Sampling Methods » Principles of Scientific Sampling

What you'll learn this session

Study time: 30 minutes

The principles of scientific sampling in psychology
Different sampling methods: random, systematic, stratified, opportunity and volunteer
Advantages and disadvantages of each sampling method
How to evaluate sampling methods in psychological research
Real-world applications of sampling techniques

Introduction to Scientific Sampling

When psychologists conduct research, they can't usually study everyone in a population. Instead, they select a smaller group (a sample) to represent the wider population. The way researchers choose this sample is crucial for making sure their findings are reliable and can be applied more broadly.

Key Definitions:

Population: The entire group of people that researchers are interested in studying.
Sample: A smaller group selected from the population to take part in the research.
Sampling: The process of selecting participants from a population for a study.
Sampling frame: A complete list of everyone in the population from which a sample can be drawn.
Representative sample: A sample that accurately reflects the characteristics of the population.
Sampling bias: When certain groups are more likely to be selected than others, making the sample unrepresentative.

Why is Sampling Important?

Imagine you want to find out what teenagers in the UK think about social media. You can't possibly ask all teenagers in the country! Instead, you need to select a smaller group that represents UK teenagers as a whole. The way you select this group will affect how reliable your findings are.

Real-World Example

In 1936, a magazine called Literary Digest conducted a poll to predict the US presidential election. They sent surveys to people on magazine subscription lists and telephone directories. Their sample predicted that Alf Landon would win by a landslide. In reality, Franklin D. Roosevelt won by a huge margin! The problem? Their sampling method was biased - in the 1930s, only wealthier people had telephones or magazine subscriptions and they tended to vote differently from the general population.

Types of Sampling Methods

Psychologists use several different methods to select their samples. Each has its own strengths and weaknesses.

🎲 Random Sampling

How it works: Every member of the population has an equal chance of being selected. Like picking names out of a hat!

Advantages:

Reduces bias - no one is more likely to be chosen than anyone else
Creates a representative sample
Findings can be generalised to the wider population

Disadvantages:

Needs a complete sampling frame (list of the population)
Can be time-consuming and expensive
May still miss certain groups by chance

Example: Using a computer to randomly select 500 students from a school register to complete a survey about exam stress.

📊 Systematic Sampling

How it works: Select participants at regular intervals from a list (e.g., every 10th person).

Advantages:

Simple to use
Spreads the sample across the whole population list
No need for random number generation

Disadvantages:

Can create bias if there's a pattern in the list
Still needs a complete sampling frame

Example: Selecting every 20th person from the electoral register to take part in a study about voting behaviour.

👥 Stratified Sampling

How it works: The population is divided into specific groups (strata) based on characteristics like age, gender, or ethnicity. Then participants are randomly selected from each group in proportion to their size in the population.

Advantages:

Ensures all important subgroups are represented
More accurate representation than simple random sampling
Good for comparing different groups

Disadvantages:

Need to know the proportion of each subgroup in the population
More complex and time-consuming
Requires more planning

Example: In a school of 1000 students (600 girls and 400 boys), selecting 60 girls and 40 boys for a study to maintain the same gender ratio.

🚶 Opportunity Sampling

How it works: Selecting people who are easily available and willing to take part (also called convenience sampling).

Advantages:

Quick and easy
Low cost
No need for a sampling frame

Disadvantages:

High risk of bias - not representative
Limited generalisability
Some groups may be over-represented

Example: A researcher studying shopping habits by asking people who happen to be in a shopping centre on a Tuesday morning.

✋ Volunteer Sampling

How it works: Participants choose to take part in the study themselves, often responding to advertisements or requests.

Advantages:

Participants are usually motivated and engaged
Can reach specific groups who are interested in the topic
Relatively easy to implement

Disadvantages:

Self-selection bias - only certain types of people volunteer
Not representative of the wider population
Findings have limited generalisability

Example: Posting an online survey about mental health on social media and analysing responses from those who choose to complete it.

Evaluating Sampling Methods

When deciding which sampling method to use, psychologists consider several factors:

✅ Representativeness

How well does the sample reflect the wider population? Random and stratified samples tend to be more representative.

💲 Cost and Time

Random and stratified sampling take more time and resources. Opportunity and volunteer sampling are quicker and cheaper.

📝 Research Aims

Some studies need representative samples, while others may focus on specific groups where convenience methods might be appropriate.

Sample Size

As well as how participants are selected, the number of participants (sample size) is also important:

Large samples generally give more reliable results and reduce the impact of individual differences.
Small samples are easier to manage but may not represent the population well.
The ideal sample size depends on the type of research and statistical analysis being used.

Case Study Focus: Milgram's Obedience Study

In his famous obedience study (1963), Stanley Milgram used a volunteer sample of 40 male participants who responded to a newspaper advertisement offering $4.50 for taking part in a study of 'memory and learning'. This created a biased sample as it only included people who read that newspaper, were motivated by the payment and were available during the study period. The sample was also all male and from one geographical area (New Haven, Connecticut). This limits how widely we can apply the findings to other groups or cultures.

Ethical Considerations in Sampling

When selecting participants, psychologists must consider ethical issues:

Informed consent: Participants should understand what the study involves before agreeing to take part.
Voluntary participation: People should not feel pressured to participate.
Right to withdraw: Participants should be able to leave the study at any time.
Confidentiality: Personal information should be kept private.

Applying Your Knowledge

When you read about psychological studies, always look at how the sample was selected. Ask yourself:

What sampling method was used?
How might this affect the results?
Can the findings be generalised to other groups?
What biases might exist in the sample?

Understanding sampling methods helps you critically evaluate psychological research and understand why some findings might not apply to everyone.

🧠 Test Your Knowledge!